Mech Engineering/Machining/Feasibility Question

[thk2c]

I have a question for all you Mech Engineers/ physics nuts.

I want know if it is possible to safely fabricate one of these

http://www.petzl.com/us/pro/verticality/anchors/rigging-equipment-and-mobile-life-line-0/paw

I have access to aluminum scrap and a CNC machine and have the basic skills to draw it out in a CAD program. Beyond that I don't know anything. So, I was hoping you could help answer some questions.

1. Strength. There must be different grades of aluminum. What has the best strength/weight ratio? Are there post milling procedures I can use to increase strength?

2. Safety. Am I missing something here? Assuming I copy the same design and use the proper grade of aluminum, would it be safe? Is there any way I could calculate a theoretical max load capacity per my CAD drawing and aluminum grade?

3. Suggestions/Comments/accusations of insanity... I am open to hear them all.

Answers to questions you might pose.

1. This is not for rescue/climbing purposes. More than likely it will be used to set up long slacklines (thereby removing any chance I would ever use it for climbing) and rigging light jobs (i.e. rigging lines to lift the bed out of a pickup truck to replace the fuel pump).

2. How strong do I need it to be you ask? Too strong. While this isn't for climbing purposes, slacklines do generate quite a bit of force. The weakest piece of equipment I use now (the line itself) has a max force rating of 17 KN, so I would like the rig to be able to handle at least that amount force (although, I doubt it will ever see anything remotely close to that).

3. If I had the money, I would buy them from Petzl. Thing is I don't have the money ($200 for two). I do have the materials and some time though, hence the original question. Plus I enjoy building things, I don't enjoy spending money.

Thanks in advance for any input you can give!

Tyler

 

[MoonRise]

Aluminum can range in strength from about equal to a stick of butter up to about a 'medium' strong steel.

So, regarding the 'strength' of aluminum, it depends.

Next, aluminum is quite useful but like most things in life there are trade-offs. If you don't need some of the positive trade-offs from aluminum then skip using it so that you avoid some of the negative trade-offs.

Since you don't need 'crazy lightweight' like climbers do, then skip the aluminum 'rigging parts'.

Next, also skip using rope to lift things over you (like a truck bed in order to replace a fuel pump). Use lifting-rated items. Chain, wire-rope/cable, lifting slings, etc.

IMNSHO.

 

[east_tn_emc]

Mechanical Engineer here......yes, you could certainly machine and make something that looks just like that....likely even out of the same material....but can you do the same testing of the base materials before it is machined....the same Non-Destructive-Testing that is done DURING the machining process.....and the same testing AFTER it is made to make sure it has the correct strength and properties that it is supposed to have and was designed to have?

If you cannot test to make sure the widget has the proper characteristics it was designed to have, I would not want to be around when you used it.

Just my 2-cents worth....

 

[rsanter]

yes you can
I would use 6061 or 7075 alm
but
keep in mind that the liability then is on you.
I think if I was making them myself I would concider using steel

bob

 

[thk2c]

Thanks for the input!

Moon Rise and East TN EMC, that was what I was afraid of. I am glad I asked before putting the time and effort into building it.

Also, steel is out as a material. Aluminum carabiners on steel is a no-no. It will tear them to pieces and I don't want to have to go out and buy steel biners.

Maybe they will go on sale somewhere.......

 

[Zengineer]

My engineering opinion based on the type of questions you are asking...

You're in over your head if there is any chance of injury as a result of using these items.

 

[deranged]

I think if I was making them myself I would concider using steel

Agreed, another ME here. steel would be stronger and for the size the weight would still be negligible.

 

[GarageEnvy]

Not an engineer but I used a lot of Petzl stuff back in my climbing days. Another vote for steel. Probably the only reason they were not steel is for weight. Most all climbing stuff is rated much higher than you'd ever conceivably load it. Look at the side of a "biner" some day. Used climbing stuff goes for next to nothing because most climbers won't buy it. You might try and find a used one before you reinvent the wheel. But if you do, go steel. It doesn't seem like the weight savings would be important for your use.

Also, if you google Petzl PAW you'll find lots of retailers selling this for $27.95 to $52. Still want to DIY this?

 

[charle10]

Why do you need something like that for making a slackline? Aren't there knots that would function just as well? I also agree with the previous posts (using for correct purpose, and testing). Creating up to 4000lbs (17KN) force on a slackline doesn't take a whole lot of weight, but its still more than enough to hurt or squish someone real good. Be careful.

 

[unclemoak]

That would be stupid simple to make.

1. 10 minutes to draw in CAD
2. cut on waterjet
3. deburr
4. ???
5. profit

 

[Ign]

4. ???

4. anodize

 

[bochnak]

Also, if you google Petzl PAW you'll find lots of retailers selling this for $27.95 to $52. Still want to DIY this?

x2. I find most things are easier to buy than to make myself if readily available and not custom.

 

[asp]

ME here... I don't claim to have all the answers, but perhaps a bit of insight. I have fabricated many of my own parts both for my Jeep and for other projects. Some things have been steel, some have been aluminum.

The bad engineer is the one who overflows at the mouth on 'just do this then this then this and voila you have your product.' We all know it's more complicated than that, but if I were to make my own, this how I'd do it.

Copy their dimensions, figure out what material they use, then make it 10-20% thicker just for good measure.

East TN EMC makes a good point about materials testing. Were I an engineer at Petzl, I'd want to do the same tests. For my own use I would probably accept nominal values of yield strength for that material and do my design with a large factor of safety, as I'm sure Petzl has done too.

Take a look at matweb.com for a good starting point on material strength and stiffness. Do some quick math on approximately how much force will be applied to each hole and using the cross sectional area figure out the stress in the section. Don't forget about the fence post rule (3 holes, 4 posts) when considering the cross sectional area. You never ever ever want to come close to the yield stress of the material during operation for anything that's designed to support a human.

As stated, aluminum has vastly differing properties based on what grade it is. Knowing what they use is very important. I don't think I'd just grab something from the scrap pile unless you knew what it was or where it came from. I'd even do some materials testing of my own if I had access to that too.

edit: I'd make my own if it were $200. I wouldn't make my own if it was $25-$50.

 

[Zengineer]

No mention of fatigue life yet... by the engineers or non! Your ideal design stress should at least be considering fatigue life with an aluminum product. Steel gives much more leeway.

See my previous statement.

 

[Steve from Socal]

I am not an engineer but I did major in physics! Using fibrous lines such as rope require careful consideration. As every sailor know chafing is a serious consideration. The radius of the holes would be a significant issue. If you are dead set on rope I would strongly suggest pulleys in lieu of the devices shown. As far as pulling a truck bed for access I would consider a lifting sling and a come-along far more controllable.

Steve

 

[IndyGarage]

So I'm a mechanical engineer, and I believe you've gotten some good advice so far, but I'm going to say this - you could easily fabricate one of those things without a CNC machine.

If I understand what it does, there's no real critical dimensions on it. Just get some very high quality aircraft grade aluminum stock (DO NOT USE THE STUFF THEY SELL AT THE LOCAL HARDWARE STORE! I think they carry the good stuff on aircraft spruce), lay the holes out with a set of drafting tools, drill the holes with a drill, cut the scallops with a jig saw and a belt sander, and then use a carbide router bit to round over all the edges.

I'm pretty sure the anodizing makes the surface a slight bit harder - but is more for looks than anything else.

The one thing you don't know about it is whether it has any heat treat done to it - so you need to be sure about whether it is hard aluminum and what grade it is. Sometimes the manufacturer will come right out and say what material it is.

I'd say if you went with 6061 or 7075 T6 you'd probably be OK, but as several point out there is no way to be sure until you do the analysis. My engineering handbook says 6061 T6 is about 35,000 psi to yield - which is roughly 1/2 the strength of steel for any given dimension. Of course Aluminum weighs much less than steel - a little more than 1/3 as much for a given volume.

So here is an example load calculation:

I'm completely guessing, but it looks like the smallest dimension across solid aluminum on those things is no less than 3/8 inch. If the thinkness is 1/4 inch, then the minimum cross section would by 1/4 x 3/8 or .0938 square inches. Now multipy .0938 square inches times 35000 pounds per square inch, and you get 3281.5 pounds of strength minimum.

However, you cannot load it up that much! - That is the theoretical point at which it will break, and factors such as improper machining, gouges, base metal imperfections all reduce that number

So then you reduce that amount by a "safety factor", which is almost always at least 1/2 - but to be safe should be at least 1/4. So, divide 3281.5 by 4 theoretically gives you the safe load - of about 820 lbs for this device

Hope this helps.

 

[IndyGarage]

So, I went back and looked at the PDF on the product, and they show a MBF (Minimum Breaking Force) of 36 KN - which equates to 8,000 lbs - much higher than the 3200 lbs I calculated.


Something fishy here: It doesn't look to be more than 3/8 inch thick, and I can't imagine them not being really, really conservative on the numbers... Maybe my engineering education is outdated - or maybe they simply allowed the CAD system to do the calculation for them - applying a load in the vertical direction only as they show on the brochure. If they did this, it's a pretty aggressive engineering calculation that I've seen before. The CAD system will do the calculation correctly, but since the load is applied in precisely the vertical direction - it will use the strength of two cross sections and add them together - essentially doubling the calculation I made above. (which makes the numbers start to jive again). Technically this is a correct calculation, but advertising wise, it's a big mistake, because, in real use, it's almost impossible to load the thing in a perfect vertical plane.

 

[MoonRise]

No mention of fatigue life yet... by the engineers or non! Your ideal design stress should at least be considering fatigue life with an aluminum product. Steel gives much more leeway.

See my previous statement.

See my previous statement also ...

...
Next, aluminum is quite useful but like most things in life there are trade-offs. If you don't need some of the positive trade-offs from aluminum then skip using it so that you avoid some of the negative trade-offs.

...

IMNSHO.

Buried in the "negative trade-offs" statement would be such properties of aluminum as a finite (never an essentially infinite, as steel CAN have) fatigue life, possible notch sensitivity, possible internal grain/inclusion defects serving as crack-initiation sites (related to an even further reduced fatigue life or a lowered stress design limit from 'perfect' condition material), possible material wear from the (generally) softer aluminum, lower modulus of elasticity causing greater "elastic" deformation than steel (1/3 lower modulus), a generally much 'sharper' transition from elastic to yield to failure for aluminum than for steel, etc, etc, etc.

That would be stupid simple to make.

1. 10 minutes to draw in CAD
2. cut on waterjet
3. deburr
4. ???
5. profit

Left out a little bit about testing and analysis and liability there.

As to the OP, -could- you make such a thing? Sure, you 'could'. But 'should' you? No, IMNSHO.

IndyGarage, 7075-T6 has a yield strength of ~60 ksi (note: I do NOT know what material the Petzl PAW is made of) but an ultimate tensile strength of ~71-73 ksi. If Petzl is listing the MBF, then I think they may be going by ultimate tensile strength (complete failure mode) and not by the yield strength. Using a UTS of 71 ksi and an 8000 lb 'strength' gives a calculated required cross-section area of just 0.113 in^2. That completely ignores any and all stress-concentration factors and factors of safety, and fatigue is not calculated for ultimate tensile strength conditions. Real-world 'safe' working load limits would typically be approximately 1/3 to 1/4 of the UTS, or lower for 'life-safety' conditions (overhead lifting or lifting/rigging of personel).

Also note the listed 'fine-print' in the PDF that the device has an essentially 'unlimited' shelf-life but is subject to normal wear-and-tear in use and must be inspected before use and a minimum of every 12 months and must be condemned/retired if the usage history is unknown or suspect or if it fails any inspection or has been subject to any "falls" (shock load conditions) or major loads (or 'overloads') or if there is "any doubt as to its integrity" (direct product quote).

The product lifetime may be reduced to a SINGLE use (emphasis added) if subjected to "chemicals, extreme temperatures, sharp edges, major fall
or load, etc." (exact quote again)

Aluminum is handy and useful. But it has pluses and minuses.

And lifting/rigging/climbing have just a few safety concerns as well.

 

[GarageEnvy]

I know what you guys are saying about the rating on this piece of equipment but in reality it is almost irrelevant. I can't think of a use for this in climbing where it would even be close to the weakest link in the chain.

 

[Charles (in GA)]

There are lots of better ways to lift the bed off a pickup.

What kind of truck? Full size? small truck?

About a half dozen people can easily lift the bed off a full size truck. My Ranger, I put two 2x6's across the bed, just under the bed rails in the middle. Wrap a chain around the boards (trailer safety chain works well) and use an engine hoist/cherry picker/ portable engine crane/ whatever you want to call it, and pick up the bed by myself. Easy.

Charles

 

[bimmertim]

I used to do bodywork for a Chevy dealer. It would take 4 of us minimal effort to lift off a full-size truck bed.

 

[IndyGarage]

Buried in the "negative trade-offs" statement would be such properties of aluminum as a finite (never an essentially infinite, as steel CAN have) fatigue life, possible notch sensitivity, possible internal grain/inclusion defects serving as crack-initiation sites (related to an even further reduced fatigue life or a lowered stress design limit from 'perfect' condition material), possible material wear from the (generally) softer aluminum, lower modulus of elasticity causing greater "elastic" deformation than steel (1/3 lower modulus), a generally much 'sharper' transition from elastic to yield to failure for aluminum than for steel, etc, etc, etc.

I don't think a piece of aluminum like that, made from proper material, would ever reach the fatigue life limit in climbing use. People fly every day on aircraft made of similar aluminum that has been stressed through hundreds of thousands, maybe millions of cycles, with no failures.

IndyGarage, 7075-T6 has a yield strength of ~60 ksi (note: I do NOT know what material the Petzl PAW is made of) but an ultimate tensile strength of ~71-73 ksi. If Petzl is listing the MBF, then I think they may be going by ultimate tensile strength (complete failure mode) and not by the yield strength. Using a UTS of 71 ksi and an 8000 lb 'strength' gives a calculated required cross-section area of just 0.113 in^2. That completely ignores any and all stress-concentration factors and factors of safety, and fatigue is not calculated for ultimate tensile strength conditions. Real-world 'safe' working load limits would typically be approximately 1/3 to 1/4 of the UTS, or lower for 'life-safety' conditions (overhead lifting or lifting/rigging of personel).

Also note the listed 'fine-print' in the PDF that the device has an essentially 'unlimited' shelf-life but is subject to normal wear-and-tear in use and must be inspected before use and a minimum of every 12 months and must be condemned/retired if the usage history is unknown or suspect or if it fails any inspection or has been subject to any "falls" (shock load conditions) or major loads (or 'overloads') or if there is "any doubt as to its integrity" (direct product quote).

The product lifetime may be reduced to a SINGLE use (emphasis added) if subjected to "chemicals, extreme temperatures, sharp edges, major fall
or load, etc." (exact quote again)

Aluminum is handy and useful. But it has pluses and minuses.
And lifting/rigging/climbing have just a few safety concerns as well.

Yes, I didn't have a figure on 7075, so 60KSI yield and 70KSI UTS would put it into the range of what's listed with my calculations.

If so, I'm betting, as the earlier post says, that this would not be the weakest link in a climbing or lifting rig. Would I sell them commercially without all the proper engineering and testing - no way. Would I make something like this for my own use and use it - yep I do stuff like that all the time.

The problem with using something like this for climbing is this: Let's say you show up for some rock climbing, and your buddy has one of these he made in his garage, and wants to use it:

• Well, are you sure that your buddy knows the difference between the aluminum stock he found in the junkyard vs. the 7075 T6 that you buy from an aircraft supply house?
• Do you know that your buddy didn't cut the holes with a torch and ruin the temper, or worse yet then tried to retemper it?
• Do you know that he didn't use this thing last week to lift an engine out of his car and forgot to unbolt the motor mount and lifted the whole car off the ground with it? Or that it's been bent into a pretzel and straitened somewhere along the way?

You don't know any of that. That's why you double check everything and pay the $200 for the Petzl that's in very good condition. It's one thing to use something like that for lifting something out of the bed of a pickup truck and a different thing entirely for hooking onto a climbing rig.

 

[Kevin54]

Piece of cake to machine them. I'd use either 6061 or 7075 Aluminum. But to use them for lifting something like the bed of a truck, I don't think I would do it that way or go to the trouble. I lifted my fullsize bed off many times when I was doing the bodywork on it using (2) 1" ratchet straps ran corner to corner in an "X", then used a cherry picker to pick it up. There is also an "H" bar that adjust, that you could either make or buy that is used to lift beds off.

http://truck-bed-lifter.brutmfg.com/images/pickup-truck-bed-lifter-main.jpg