Loading a BendPak Asymmetric Lift

[mslisaj]

I have a XPR10-ACX and it works perfectly. This is the first side twin post lift I have ever used. My question is loading in the asymmetric configuration. I have read all the directions and all the safety information. I even have the lift and loading charts for the cars and trucks that I lift so I know where the center of gravity is for each vehicle. With the heavier vehicles up to 6600 pounds I try to center the load as close as possible between the posts. I have always questioned how out of center of lift you can go. I put this question to BendPak and the response was never exceed the 2500# limit on each arm. I put up the scenario lifting a vehicle that puts 2200# on each rear arm and 1000# on each front arm. The answer was the same, do not exceed the 2500# limit per arm and I would be within the design spec's of the lift. Does anyone have an comments on this?? When you're walking under 2 1/2 - 3 tons of vehicle it's a humbling experience. As I first said the lift picks up any vehicle very easily but I have not gotten used to these asymmetric lifts. It just doesn't look right. I do check and double check the loading and I give the vehicles good shake test but I would like some other feedback on this issue.............. Thanks for your comments.

 

[pattenp]

One thing to understand is that asymmetric lifts have columns that are rotated which places the the load center somewhat behind the columns, not directly between the columns. Is there not a diagram thats shows where the center of load is on the lift?

 

[pattenp]

The lift you have does both asymmetic and symmetic. This diagram shows the lift centers. http://www.bendpak.com/files/384_716266021.pdf

 

[mslisaj]

Thank you guys for the replies. Yes I have see the pictures of that center of lift behind and also directly between the columns. But this adds to the confusion. So we can interpret that BendPak picture to mean that anywhere between those two centers is a balanced load. But if you look at my original post, BendPak answered that as long as you don't over load any arm over the 2500# load limit you're okay. Fine, I trust that but that puts a lot of weight behind the columns and my question was does anyone find that information uneasy. That means that the load is not going directly down through the columns but is a pulling motion back. I'm looking for someone to dispute that. Yes I have good anchors in the concrete but that front anchors are being pulled up it would seem and the rear ones pushed down. Again, I appreciate your thoughts there and looking for others...........

 

[Ironcrow]

Yes, your assessment of the loads on the anchors is correct. Yes, it appears BendPak is telling you that 2500 lbs on the rear pads and zero on the fronts is within design limits. Yes, that makes me uneasy.

I have a Benwil which has a substantial welded column base that is 3 feet long along the length of the car, extending mostly to the rear of the columns. The rear edge of the column base is behind the car CG. That is, if the anchor bolts should suddenly DISAPPEAR, the lift would not fall over.

 

[soarhead25]

I was a mechanic for several years and now that I have my own assymetric lift at home it concerns me a bit too, but here is what I do. I also have a bendpak assymetric by the way.
Look at the rotation of the posts. Imagine a centerline between the posts intersecting between them. I use that as a rough spot for centering the cars CG.
Yeah, it looks funny where 2/3 of the car is behind the posts but that is how it's designed.
I also bring pickups, suburbans, and minivans further forward than most cars by about 4-8". typically you will have two marks for your vehicles front tires. one for passenger cars, the other for suburbans, trucks, etc.
Years of experience lifting cars day in and day out helps alot too, but here is another trick I use....
As the arms engage the chassis/body on the way up, I look up at the top of the column to see which way it's straining, which is typically backwards, and here is where I make a judgement call of too much strain and then bring the car forward a few inches and try again til I'm comfortable with it.
Just something to try.....

 

[mslisaj]

Yes, your assessment of the loads on the anchors is correct. Yes, it appears BendPak is telling you that 2500 lbs on the rear pads and zero on the fronts is within design limits. Yes, that makes me uneasy.

I have a Benwil which has a substantial welded column base that is 3 feet long along the length of the car, extending mostly to the rear of the columns. The rear edge of the column base is behind the car CG. That is, if the anchor bolts should suddenly DISAPPEAR, the lift would not fall over.

To Ironcrow, We agree and your example of bolts "disappearing" and the lift would not fall is my idea of how it should be balanced. Trust me I don't push those designed limits but I believe in order to use something like this safely you have to know how it works and why it works........ Thanks again for taking your time to respond to my question and continued education.

 

[mslisaj]

I was a mechanic for several years and now that I have my own assymetric lift at home it concerns me a bit too, but here is what I do. I also have a bendpak assymetric by the way.
Look at the rotation of the posts. Imagine a centerline between the posts intersecting between them. I use that as a rough spot for centering the cars CG.
Yeah, it looks funny where 2/3 of the car is behind the posts but that is how it's designed.
I also bring pickups, suburbans, and minivans further forward than most cars by about 4-8". typically you will have two marks for your vehicles front tires. one for passenger cars, the other for suburbans, trucks, etc.
Years of experience lifting cars day in and day out helps alot too, but here is another trick I use....
As the arms engage the chassis/body on the way up, I look up at the top of the column to see which way it's straining, which is typically backwards, and here is where I make a judgement call of too much strain and then bring the car forward a few inches and try again til I'm comfortable with it.
Just something to try.....

Mr. Sorehead25, I appreciate you bringing your long years experience to this discussion. I do have the picture of the turned posts and the center line intersection. That allows the CG to be about 2' behind the posts. Okay, I understand that too. I work comfortably within those limits. I like your idea of watching the columns for "strain". I do that now and I believe for my safety and to work within 'my' safety limits I will keep the CG within the exact center and the 30 degree turn center. I should stay safe within those ranges. I also watch the vehicle as it just comes off the floor and see which way it wants to go too. This is an amazing piece of equipment and I so appreciate you guys that are taking your time to express your opinions and experience. You're keeping me out of trouble............
Thanks......... Lisa

 

[Ironcrow]

I should point out that many twin post lifts for the hobbyist market do not have a cross bar connecting the tops of the posts. In these cases there is a considerable force bending the columns toward each other no matter where the car is placed on the pads. Even if this makes me uneasy, this load condition is tolerated and the mounting studs do not pull out of the floor.

So, the fore/aft leaning force is safely supported.

My lift has a top cross piece. It is also anchored to the floor with poured in place threaded cups, not the usual expanding anchor in a drilled hole.

 

[mslisaj]

I have the XPR-10ACX lift and it does have the top bar. Believe me it's as solid as a rock and I have not observed ANY deflection of any of the componants but the biggest thing I have put up on the hoist was a 66 Chrysler. As I have mentioned, this lift is very solid but I wanted to ask the questions as I'm a new user of the twin post floor lifts and I want to be safe. I had originally asked BendPak for for some engineering details or SOMETHING that was more comprehensive then the users manual. The response was "Do not exceed 2500# per arm" and I would be within the design criteria. I was also assured that this lift was certified (I knew this before I bought it). I had inquired to how the lift was tested and as I wanted an explaination to how they may have overloaded the lift and it survived. But that information is unavaliable. As I mentioned before, I gather these lifts are pretty idiot proof as the instructions and key information is very brief. But trust me, I'm centering heavy loads the best I can. I have a Toyota Tercel and that car is so light that I couldn't over load the lift as long as I got the four arms set on solid metal. But I think the discussion is good and educational for all that have ever wondered.

Thanks again for your continued input.......... Lisa

 

[JSK]

Although I understand your uneasiness, rest assured your XPR-10ACX and has been tested in absolute worst case conditions and is completely safe when lifting rated loads or less.

I’ll try and explain it as best I could. You can load your lift with 10,000 pounds in whatever fashion required to safely lift and balance the vehicle as long as the CG is centered between the adapters. That means with the trucks/cars positioned all the way to the rear (yes, I know it appears strange and uncertain) or towards the front.

When undergoing ETL certification, we are required to test our 10,000 pound asymmetric lifts with the rear (longest) arms pulled all the way out (full extension) and the front arms positioned at approximately 90-degrees relative to the columns. We then load the lift with 150% of rated capacity (15,000 pounds) and run it through five full lifting cycles. Intertek inspectors then inspect the entire lift searching for any part(s) with detectable/measureable signs of deformation or wear. If any signs of deformation are evident, the lift design fails. Your XPR-10ACX is certified.

Does a vehicle balance better on a symmetrical lift than an asymmetrical lift? No it does not. The vehicles are balanced exactly the same assuming the vehicle is lifted on the four factory recommended lift points.

You need to be most concerned when lifting long wheelbase vehicles or those with cantilevered rears. I remember visiting a shop once that had a dual-cab super-duty truck loaded on a two-post lift. I would have guessed the truck to have an 8,000 pound curb weight. I noticed the rear arms sagging more than normal. Next, the heart-stopper - I noticed one of the front pads was not making contact with the vehicle. After investigating the unusual condition, I observed the tech had loaded the lift with the rear arms positioned forward on the frame. I could tell he was almost forced to as the rear part of the frame slanted upwards giving no flat surface for the pads to rest on. I then guesstimated that because the vehicle was so long and had a cantilevered rear, and because the pads were positioned too far forward, the rear arms must have been carrying almost the entire load of the vehicle. I pointed it out to the tech and he simply laughed it off as if I was worrying about something for nothing. For shits and grins, (although I shouldn’t have), I wanted to test my theory by seeing how much weight may have been resting on the front lone pad. Sure enough, by lifting very carefully on the front bumper I was able to raise the front of the truck ever so slightly. Ignorance is bliss I guess.

Jeff
BendPak

 

[mslisaj]

Wow Jeff!! That was the explanation I was looking for from BendPak from the beginning. Thank you so much! Now I get it. Your example of that big truck was truly scary for sure. You didn't mention if it was your your lift though but it really doesn't matter as the certification, testing and design took into consideration a tech that didn't know and as you story proved didn't care in the least. I have a tow truck that grosses 6600#. I axle weighed it and it's 4000# on the rear axle and 2600# on the front. I gather the reality is if I loaded this vehicle centered on the lift I could safely pick it up. But I have been servicing this vehicle on the floor with jacks and stands for years and I'm more comfortable there. But your story and explanation were EXTREMELY informative. Thank you for taking your time to put out a very thorough explanation and thanks to BendPak for a quality lift. I will be getting more used to walking under a couple of tons of car.

 

[Nutty 5.0]

Jeff is very good!

Lisa,
I look at the post (Standing from say the motor column) and picture the car centered in the middle of that when looking at it straight on. This seems to help me for the most part. Some trucks like my 02 F150 needs to be just right to reach similar to how Jeff speaks about.

 

[mslisaj]

Thanks Nutty for the suggestion. Next time I set my pickup in between the posts I try your method..............

I appreciate your response..........

Lisa

 

[Ironcrow]

When undergoing ETL certification, we are required to test our 10,000 pound asymmetric lifts with the rear (longest) arms pulled all the way out (full extension) and the front arms positioned at approximately 90-degrees relative to the columns. We then load the lift with 150% of rated capacity (15,000 pounds)

Is that 150% load distributed evenly on all four arms? That's 3750 pounds per arm. Or 7500 on the front pads, essentially all compressive load on the columns. The 7500 pounds on the rear arms is all moment, trying to twist the columns at the base with, what, a 4 foot moment arm? That's 30,000 ft lbs. Given that the column base is about 1 foot across, gives us 30,000 lbs of pull-out force. If that's divided between 4 studs, we have 7500 lbs per stud.

When you do the test, how is the lift bolted down?

I would think that the lift itself would not fail, but rather it would pull out of the floor. Thankfully, that will take 200 or 300% (?) of the lift's rated load.

 

[mslisaj]

Way to go Ironcrow!! Now we're getting into the meat of this discussion. I hope Jeff from BendPak picks up this thread now and responds to your point. One change I would make to your facts is the XPR-10ACX has Six mounting bolts to the floor for each column. But your point is well taken. It's the Length of the Arm and the Moment computation is where the action is on this lift. The fact that the floor or the anchors are more likely to fail then the lift itself is the real point. So exactly HOW was the test lift bolted to the floor is the real question? Thanks again Ironcrow for bringing up this type of question. While I'm not an engineer I am a pilot and the weight and moment and arm are all part of the Weight and Balance of an airplane and while BendPak has gone to great lengths to keep me from grabbing a calculator every time I use there lift the relevance of the weight and balance calculation can't be over looked. Again, Great job Ironcrow for asking this question.

Lisa

 

[Ironcrow]

In the absence of any other input, the generic result for an expending bolt 3/4 inch anchor is about 5000 lbs per anchor. So, you want all your anchors in and performing to spec. Personally, I don't trust them to perform to spec when they are new, let alone 10 years from now....

 

[mslisaj]

Well Ironcrow, I agree, this thread was going so good there and quite a bit of good technical information came out. I think your analysis may have ended the discussion as you were hitting right at the crux of the matter. The twisting and bending moment and arm calculations are tough for most to understand. I was going to cut and paste your comments into a private note to Jeff at BendPak as I would love for him to take that to his engineers and get their numbers and the "envelope" of calculations put up here. But I sincerely appreciate your continued input in this discussion.

To your point that each anchor is good for 5,000# +or- when they are new that would give me 30,000# of anchor force per post. So the lift is good for a max total load of 10,000# then the anchors could deteriorate all they way down to 1,000# apiece and I still should have a safe hoist that would have to be loaded at center. But I think to the real point that I or anyone else needs to understand, regardless of symmetric or asymmetric that you need to "balance" the load between all the lifting points and between the post as close as possible to get the load going straight down through the posts and distributed directly down into the floor with out a lot of twist or pull on the posts. Sure as Jeff so well pointed out that the lift is designed and will take a good over load on one pair of arms or the other but now we have to pass the discussion on to the anchors and the concrete. That truly is the only variable that even the engineers can't guarantee. While I did have my shop custom built at the time I didn't anticipate ever getting a lift. If I had I would have strengthened the floor with deeper footings where the hoist sits now and would have investigated embedded anchors at the time of the pour...........

Great discussion Ironcrow. As usually I want to thank you for your input. I think we all have contributed a lot of facts to make this discussion very complete on this subject.

Lisa

 

[kvom]

A static analysis of a post loading shows that the tortional load on the bottom plate is highest when the vehicle is just off the ground and gets less as the vehicle rises. This is counter-intuitive.

Assume a 5000 lb vehicle evenly supported on 3' arms. Each post feels a 2500x3' or 7500 ft-lbs of torque at the point where the arms meet. The torque is counteracted by the anchors/baseplate with a moment arm equal to the height of the arms above the ground. So at a lift of 1' the anchors resist 7500 lbs. Raise the vehicle 5' and the resistance drops to 1500 lbs.

Since the anchors will be on both the inside and outside of the post, the outside anchors are the only ones under tension and are providing a smaller portion of the resistant torque.

 

[mslisaj]

Could you explain that in more laymans terms? As you said it's very counter intuitive. So exactly how does that work? Also with the BendPak lift that I have there is a very substantial cross bar between the posts which resists the twisting motion going to the inside or post to post. But the fore and aft torque loads on the posts and anchors are of real interest..............

Lisa

 

[Daedalus]

Found this thread on Google while looking for Bend Pak anchor torque specs, and thought I would join and reply. The moment (torque) about the baseplate does not change with vehicle height, assuming the posts are vertical (they better be!). Moment about a given point is F x D, where F is the force (weight) vector, and D is the distance between the force vector and said point. Since the force of gravity is straight down, perpendicular to the floor, the distance between the force line of action and the baseplate doesn't change with vehicle height. Generally, stresses on a lift and on the anchors don't change after the vehicle leaves the floor (aside from local stresses in the posts and minor accelerations from starting and stopping the hydraulic rams).

I have a similar concern about loading on my symmetric lift. I'm relying on safety margins and my own calculations. I know that my garage foundation isn't quite as thick as it should be, and it scares me everytime the lift "pops" from settling. I can park with the CG between the posts, but some of the work I do requires removing engines/drivetrains, which moves the CG more than the width of the baseplate. I bias the CG as best I can.

 

[mslisaj]

Found this thread on Google while looking for Bend Pak anchor torque specs, and thought I would join and reply. The moment (torque) about the baseplate does not change with vehicle height, assuming the posts are vertical (they better be!). Moment about a given point is F x D, where F is the force (weight) vector, and D is the distance between the force vector and said point. Since the force of gravity is straight down, perpendicular to the floor, the distance between the force line of action and the baseplate doesn't change with vehicle height. Generally, stresses on a lift and on the anchors don't change after the vehicle leaves the floor (aside from local stresses in the posts and minor accelerations from starting and stopping the hydraulic rams).

I have a similar concern about loading on my symmetric lift. I'm relying on safety margins and my own calculations. I know that my garage foundation isn't quite as thick as it should be, and it scares me everytime the lift "pops" from settling. I can park with the CG between the posts, but some of the work I do requires removing engines/drivetrains, which moves the CG more than the width of the baseplate. I bias the CG as best I can.

Interesting comments Daedalus,

Then are you saying that KVOM's comments about the torque loading are wrong?? After reading these responses I have been trying to learn this stuff. Plus I'm not an engineer and don't have access to one other then you
gentlemen that may respond here. The way I was understanding this and the discussion got started with regards to the asymmetric nature of this lift and actually loading the rear arms more then the front arms and how was that possible and still be safe. "Jeff's" comments point out that BendPak designs this lift that as long as you don't overload the individual arms over the 2500# design limit you are safe. My question was how did that work and KVOM explained that actually the torque load of that type of loading was absorbed in the carriages. As he initially put it this seems counter intuitive until you understand it. Jeff, who works for BendPak gave an extreme example above in this thread of how the lift is tested and certified so the reality for me is to load the lift as carefully as I can and center the load between the columns for my own peace of mind. But at the end of the day I GUESS we're really all relying on the six anchor bolts per column to do what they were designed to do even though I don't throughly understand the engineering behind it. Thank you for your comments and I appreciate you taking your time to put in your two cents worth...........

Lisa

 

[Ironcrow]

Daedalus is correct. The twisting moment at the base is independent of the height of the lift.

 

[mslisaj]

So are you saying that what KVOM said was totally incorrect?? Now I really am confused. He was making the point that the torque load on the column anchors dissipated as the height increased. So I guess there is no simple explanation on how you can load a car asymmetrically and how with either physics or engineering the lift can do this with out brute force at the anchors into the concrete floor.
I really appreciate you guys weighing in on this discussion. I'm going to have to get an engineering degree to understand this I guess..............

Thanks again.......

Lisa

 

[kvom]

In thinking the issue further, I have to retract my analysis. Considering the post as a simple beam between two points, The reduction in force that I posited in that required to keep the beam from rotating about the the upper point (i.e., where the arms connect). However, what we need is the force required to prevent rotation about the base. This indeed does not change with height. Sorry for the confusion.

some of the work I do requires removing engines/drivetrains, which moves the CG more than the width of the baseplate. I bias the CG as best I can.

You should use one of the tall jackshafts supporting the rear of the vehicle, so that if the weight shifts to the rear then there is extra support.

 

[mslisaj]

In thinking the issue further, I have to retract my analysis. Considering the post as a simple beam between two points, The reduction in force that I posited in that required to keep the beam from rotating about the the upper point (i.e., where the arms connect). However, what we need is the force required to prevent rotation about the base. This indeed does not change with height. Sorry for the confusion.
You should use one of the tall jackshafts supporting the rear of the vehicle, so that if the weight shifts to the rear then there is extra support.

Okay, then the cloud is not so dark. We do then deal with a twisting motion at the carriage and then there is a rotational force at the base too. This force does not diminish with height then. So I gather these facts are true. There is a lot going on with these lifts to literally balance the load the way they do. They are well engineered to do this and keep us safe as users. But this is a very interesting discussion for me as I have learned a lot............. But we're still down to the fact of the six anchors at each column resisting and holding up the entire load.

Thanks for your contribution too..........

Lisa

 

[Daedalus]

You should use one of the tall jackshafts supporting the rear of the vehicle, so that if the weight shifts to the rear then there is extra support.

It's trickier than it sounds. What I really want is to take out just a portion of the load on the arms...just enough to move the CG back between the baseplates. Once the frame makes contact with the stands, I won't know how much of the load they're taking. It's possible they would take out most or all the load on the arms on that end, especially if there's any settling. If that happens, then I have the same problem, except the torque on the lift would be going the other way. What would be ideal is a long spring that could be adjusted to provide about the right amount of force within +/- 1 inch of displacement.

Okay, then the cloud is not so dark. We do then deal with a twisting motion at the carriage and then there is a rotational force at the base too. This force does not diminish with height then. So I gather these facts are true. There is a lot going on with these lifts to literally balance the load the way they do. They are well engineered to do this and keep us safe as users. But this is a very interesting discussion for me as I have learned a lot............. But we're still down to the fact of the six anchors at each column resisting and holding up the entire load.

Exactly. The lifts are very well built. I worry about the lift failing about as much as I worry about cables above an elevator breaking. In my case I'm worried about the anchors, but if you meet all the specs defined by the manufacturer then you'll be fine. They're careful to define minimum concrete thickness, compressive strength and setbacks. The anchors used also have their own ratings and requirements.

 

[mslisaj]

Hi Daedalus,
Well you certainly want to keep the load over the base plates and this is what precipitated this discussion. How far away from the base plates can we safely move the load. According to Jeff, we can safely have the rear arms extended to their limits and up to 2500# on each arm and still be with in the balance safety limits of the lift. While this type of loading makes us all nervous, as long as our concrete meets specs and the anchors are tight the lift is designed to handle this imbalance. Now on the other hand I have tall jack stands for just the situation you are bringing up here but it's interesting how all this takes us back to the base plates and anchors.................

Lisa

 

[Skyking1992]

Lots of good information here. I have a two post 10,000 lb lift, certified and american made. The heaviest and longest vehicle that I have lifted so far is my 3/4 ton Suburban. It seems like there is a lot of truck hanging out the "back" of the lift. For my own peace of mind, I always use a stand under the rear end or frame before I do any work underneath. This might be overkill, but I feel a whole lot better.

 

[mslisaj]

I must agree I use stands too if I have my big cars or pick up on it. I also try to center the vehicle the best I can. These twin post lifts are amazing feats of engineering for sure. But we're still down to the anchors and concrete and that is truly amazing what goes on down there too...........

Thanks for your input......

Lisa

 

[tyrell2004]

Although this is a slightly different two post application, this might help for your "numbers" guys.
Showing some basic calcs for a max jack movable two post anchor bolt calculation. Showing all them vectors and moments with circles and arrows and stuff. Maybe that can steer you all towards a more definitive answer.

Don't claim to understand em, or know if they are correct (had a rally bad pre-calc prof in eng school, but that's another story), but maybe it helps.

http://www.maxjaxusa.com/specifications.html

 

[mslisaj]

Although this is a slightly different two post application, this might help for your "numbers" guys.
Showing some basic calcs for a max jack movable two post anchor bolt calculation. Showing all them vectors and moments with circles and arrows and stuff. Maybe that can steer you all towards a more definitive answer.

Don't claim to understand em, or know if they are correct (had a rally bad pre-calc prof in eng school, but that's another story), but maybe it helps.

http://www.maxjaxusa.com/specifications.html

While I admit that I'm not an engineer and this is why I started this thread, I so appreciate this kind of information. I just wish BendPak would have provided this info when Jeff weighed in on the subject. Maybe some of the guys that added to this thread will interpret what all this means. I commend MaxJax for providing this kind of info right up front. I have the utmost faith in my BendPak and thoroughly believe I bought a quality and well designed lift. I would just love to know how it works and the safety margins other then it lifted 1.5 times it rating and didn't suffer damage.

 

[Daedalus]

1.5 is the best, most useful number you can get. It means you have a 50% safety margin above and beyond the absolute worst case loading condition for the lift (10,000 lb vehicle, biased by the max arm extension possible). This suggests you could raise at least a 15,000 lb vehicle in this configuration before the lift fails. If you lift a 5,000 vehicle your margin goes to 200%. Bias the vehicle more toward center, and the margin might go up even more.

Anchor bolt stress calcs, as shown on the MaxJax page,aren't hard to do, but the numbers suggest a lift will fail long before the anchors will. Granted, this is for a lighter-duty, symmetric lift. But I only care about the safety margin on whatever's going to fail first. Everything else is overdesigned.

I will reiterate that if you are still worried about overturning moment on the lift (as opposed to total weight on the lift), then you absolutely should not place stands under one end of the vehicle (unless you leave a small gap or otherwise know how much load is being applied to the stands). If 2 identical twins are balanced on a see-saw, and one child suddenly jumps off, the load on the see-saw is reduced by 50%, but the moment about the pivot momentarily goes from zero to the max possible, given the weight of the child and the length of the lever.

 

[mslisaj]

Hi Daedalus,
Thanks to you gentlemen I'm getting a real education here. I certainly understand every word you said and you make a good point about the tall stands too. When you think of it in the teeter toter scenario it makes great sense. When I use the stands at all the vehicle is already on the safeties of the hoist so that is solid and I just put the stand to the car. It's not supporting any weight but it's just the safety margin if I removed a heavy component and changed the CG enough to make a difference. But the reality is to your point about the weakest link. I know the lift can handle the load and I center the load the best I can and go through all the steps to make sure it's right so I'm not really worried. I just want to understand how it works. I think if this was a symmetrical lift and you were told to load everything as close to center as possible I wouldn't have asked the question as the answer would be obvious. But when the Asymmetrical part is thrown into the equation and it's advertised to load to the rear of center, it's like a magician, I want to know how that was done. Thanks to you and some others here you have exposed the magician and the trick. I just wanted to understand the physics of what was going on. The reality is this all still comes down to the anchors and cement. That is the only variable and I'm dealing with a new concrete and rebar reinforced floor so I don't have any worries. But I won't be foolish and will load every vehicle like it was the first one and check and check again that it's right as now I know how this lift truly works and I am confident I can use it safely...........

Thank you Daedalus for your consistent input. It's very much appreciated by me and others that will read this thread.........

Have a fine day......

Lisa

 

[Ironcrow]

...Don't claim to understand em, or know if they are correct...

http://www.maxjaxusa.com/specifications.html

I understand them and the calculations are correct. Interesting points for our discussion are some of what is NOT said:

1) The torque twisting the base is 11,000 foot pounds (!). In this example the torque pulling the top of the columns together. In my lift, this is supported by a cross-beam connecting the columns. The example lift has no cross-beam. Calculation technique is identical for rocking back of an asymmetric lift. This axis not considered in the symmetric loading.

2) Notice the height of the lift arms is not given or considered in this example. The load is calculated independent of height. As we said, the twisting force is not changed with the height.

3) In the calculations, the bending and breaking strength of the steel columns and lift arms is not even mentioned. What is the engineer telling us by this omission? The lift structure itself is indestructible. If it falls, it will rip the anchor bolts out of the floor. This is the weak point.

4) To achieve the calculated safety factor of 400% the rated pull-out of the anchors MUST be achieved at 12,580 pounds. My lift has set-in-concrete anchor sockets. If I was forced to mount another lift with drill and set anchors, I would consider epoxy as well as expanding bolts. I would also test each and every anchor at, say, 10,000 pounds. Well equipped contractors who do this kind of work have calibrated hydraulic jacks specifically for verifying anchor pull-out for governmental inspectors. Even if not required by any law, I would obtain the services of such a contractor.

 

[mslisaj]

As usual Ironcrow you have brought some very interesting comments and information to this thread. I think while eveything in this recent post is of interest #4 is very informative. I didn't realize that there was a way to test and certify the anchor bolts. I guess only the government has the equipment to do that but there is the proof. I don't guess you could make this test now that the lift is installed but again, this entire discussion boils down to the anchor bolts and the concrete. For us everyday lay people I guess we just have to figure that BendPak and other lift manufactures, like everything else in the lift have over sized and designed the six anchor bolts that hold each post of my lift. They have all the faith in the world in their structure and they accept the only variable that they have no control over and that's the floor. So they state minimums of thickness and strength of the concrete and then I would think that they have used the bolts they used because they are designing something with excess safety factor. Would it be safe to say that maybe in a real world with the government test system that we could get away with smaller bolts to do the job? Maybe they use such big bolts to over engineer the anchors so maybe other things would bend or fail before the lift pulled out of the floor?? With all the feed back put forth here and even the extreme example that Jeff supplied in this thread no one EVERY points to a story or event where the anchors or concrete fails under over load or unsafe usage.

With all the discussion that has been put forth here I have a real good understanding of how the lift works and it's safety factor. The next obvious question then is does anyone have stories or pictures of the concrete anchors EVERY failing?? You can go to YouTube and see pictures of cars coming off lifts but the lift anchors didn't fail the lift was over loaded or loaded incorrectly.

So I may leave that question hanging here now. Has anyone ever seen the anchors or the concrete fail and a lift fall?? Any lift??

Thanks again Ironcrow for your major contributions to this thread........

Lisa

 

[Underdog]

4) To achieve the calculated safety factor of 400% the rated pull-out of the anchors MUST be achieved at 12,580 pounds. My lift has set-in-concrete anchor sockets. If I was forced to mount another lift with drill and set anchors, I would consider epoxy as well as expanding bolts. I would also test each and every anchor at, say, 10,000 pounds. Well equipped contractors who do this kind of work have calibrated hydraulic jacks specifically for verifying anchor pull-out for governmental inspectors. Even if not required by any law, I would obtain the services of such a contractor.

I have a Bend Pak XP10 10,000# two post lift I installed myself two years ago. I used the BP supplied wedge anchors that went into a new 6" 3000psi slab. I use the lift to raise my Ford E-350 diesel van mostly. It weighs right at 7500# loaded. Never had any problems or concerns doing it, pretty much try to center it up so the cg is between the posts. I think you guys are worrying way to much about the lifts failing or the anchors pulling out. There are a ton of lifts installed out there and I haven't heard any problems as long as you don't overload em or go pulling on fasteners with a 6' cheater pipe.

 

[mslisaj]

Thanks for your input Underdog. I was asking the questions for myself as I have the minimum 4" 3000# concrete floor. It does have Rebar in it but I was just curious how much safety margin is built into this entire lift system. I center my loads and am very safety conscious with this lift. But your floor is quite a bit thicker and that gives you an extra edge. Do you need that much floor? For piece of mind, sure. But structurally maybe not so that is why I asked so you gentlemen could help me understand how all this worked. I must say you all have done a really good job.........

Thank you........

Lisa

 

[tyrell2004]

I think this is the type of testing equipment one might get an concrete eng to use if your floor is in question or if your slab is keeping you up at night.


http://www.ndtjames.com/catalog/strengthtesting/anchorTestSystem.html

 

[saabman]

I too found this forum googling "lift installation." I will soon be installing a Bendpack XPR-9F in my home workshop/garage. Daedalus is spot on about the force at the attachment point (on the column) of each arm. You want to load the arms for equal force rather than equal weight. First, Bendpack confirms that their symetric lifts (parallel columns) can do asymmetric or symmetric loading. Even the XPR-9F (bottom plate) can operate in this mode. IMHO, this is the mode to use most of the time. Take for example, front arms that are half as long as rear arms. The force on the column is a linear function of the arm length and pad weight, so it only takes half as much weight on the rear arms to equal the force on the front arms. Of course this is an over simplification because the force vector in the X-Y plane has different angle relative to the columns. In theory then there are complex forces acting to twist the column. So in an ideal world you would want the incident angle of the arms to be nearly the same and the effective force at the column to be the same. There is another thread with a video that shows equal arm length being used on a Mustang without the engine in it. Now that may be fine if the center of mass is dead center, but I would think that there is a missconception of symmetric arm loading if this is not the case.

Curiously, no instruction is given about left right centering. The old sports cars I work on have offset longitundonal power plants (they sit closer to the passenger side, this to compensate for the drivers weight). Empty the weight is aymetric around the fore/aft axis.

Chris