Making a floorplate type 2 post lift more stable

[Ironcrow]

These documents may have some definitive information. Then again, they might not. They should, at least, explain the standards that these lifts are tested and certified to:

http://www.autolift.org/ali-store/ansiali-alctv-standard-2011/
http://shop.bsigroup.com/ProductDetail/?pid=000000000030176083

I have received the ALI Standard in the mail. It is an expensive and copyrighted document so I should not make it available here. However, some pertinent points: Lift structure is designed generally with SF of 3. Hydraulic lines SF of 4. Fasteners SF 4. Proof load is 150% cycled up and down two times. Load is applied by rectangular plate of certain dimensions. Test is done twice, once with front arms fully extended, and if necessary with the rear arms fully extended if the two arm conditions are not met with one lift with standard load plate. There is nothing for actually calculating the fore/aft cg and loading asymmetrically beyond what is achieved with the specified arm positions.

There is a lateral asymmetric load that is pretty extreme - 62.5% to 37.5% (at 100% not 150%). I need to look a little closer at the standard to see where these odd numbers come from and what they are after here. Failure is tilting 3 degrees if memory serves. I think the idea here is to challenge the safety latch system, if one side catches and the other doesn't.

There is nothing about floor anchors other than they should be specified and tested in accordance with instructions. There is an implication, I guess, that they conform with the SF of 4 (?), although I need to look closer at this when I have the time. This makes some sense, ALI is not a floor anchor company. On the other hand, a lift designed with one mounting bolt in the center of a 6 inch square baseplate would be a poor lift design no matter what anchor was used.

 

[Diesel Dan]

As long as the anchors aren't constantly loosening you're fine.
It is expected there wll be some deflection as the columns are loaded since they are not in a direct compression state.

 

[Pwrgeek]

Can you propose an example? If the lift columns are further constrained, what other part might be unusually stressed and how? And this part would be more stressed than if the columns were left alone?

For one the lift coulmn (especially near the top) would be experiencing more sheer stress. With the column unbraced the back of the top of the column is under vertical tension as it resists the moment of the arms. With it braced it has not only this stress but also the sheer of the brace acting on it.

This does not sound reasonable to me at all. Would this bow be designed to accommodate column flex at maximum lift capacity, empty, or mid range? Wouldn't the arms move enough to accommodate column flex anyway? Raise the lift, lock the arms, and see how much the pads can be moved. This slack would seem to handle column deflection. I checked three lifts I have access to, a Rotary, Challenger, and Benwil. They were all straight columns.

The columns are straight but are the guideways that the carriage rides on?


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[Ironcrow]

For one the lift coulmn (especially near the top) would be experiencing more sheer stress. With the column unbraced the back of the top of the column is under vertical tension as it resists the moment of the arms. With it braced it has not only this stress but also the sheer of the brace acting on it.

Thank you taking the time to answer my question. I really do want to understand the engineering reasoning of my "detractors". If not agreement, perhaps merely bringing our differences (in judgement, constraints, risks etc - hopefully we can settle on the same engineering "facts") into focus is best served by a mutually respectful exchange.

On my own lift, the column is sized to resist a large bending moment at the bottom, resist vertical column buckling, and of course provide a channel in which the carriage runs. Together these constraints make the column greatly oversized for shear stresses introduced at the top of the column. I have not closely examined other lifts, but I think this outcome is common with them.

The columns are straight but are the guideways that the carriage rides on?

Also straight. In each one I looked at, the carriage rides on polymer pucks inside the column. No additional guideways were added to the inside of the column. The cross section of the column appeared to be consistent from one end to the other, straight inside and out.

I looked online to see if I could find a column/carriage designed differently and had no luck.

 

[wssix99]

I really do want to understand the engineering reasoning of my "detractors". If not agreement, perhaps merely bringing our differences (in judgement, constraints, risks etc - hopefully we can settle on the same engineering "facts") into focus is best served by a mutually respectful exchange.

I find this a very interesting discussion. It really proves that assumptions are everything. One could adopt assumptions that are so conservative that the very concept of a 2 post lift could probably be made economically inconceivable. (It's a wonder how we ever got to the Moon!)

So, here's one to ponder: Would it be informative if a lift engineer were to hop in to the thread to confirm how engineering assumptions are made for these things or would it ruin the fun/adventure?

 

[tim9lives]

I would NOT attach cables. Just make sure that the bolts securing the posts to the base plate are grade 8 bolts. FWIW...I have used many Benwil lifts similar to yours...with the base plate. On heavy lifts...They will move in a slight amount. That's normal.

 

[davejo]

Here is a video walkaround of the 15k lift I mentioned upthread. The baseplate, columns and lift arms look identical to my 12k lift. The (apparent) difference between the two is that the 15k lift has a crossbar support bolted to the column tops which also allows the equalization cables to run over the top to create a clear floor situation.

Sure, we would love to hear from a lift design engineer. For me, part of the fun of buying and using machines is the process of learning the ins and outs of how and why they work. The more I am learning about these lifts in general, the more confident I am in using them in a safe manner.

 

[Diesel Dan]

From what I could tell when I upgraded from a 9/10k floor plate model to a 10k clear floor the columns looked identical. A cross bar over head could be added easily.
My new 10k worth dwarfs the new 12k rotary they have at work.

 

[Ironcrow]

Here is a video walkaround of the 15k lift I mentioned upthread. The baseplate, columns and lift arms look identical to my 12k lift.

From what I could tell when I upgraded from a 9/10k floor plate model to a 10k clear floor the columns looked identical.

Yes, I think this is also common. My Benwil lift appeared to have the same column as the next capacity up. Makes sense. Once you've engineered the bigger one, it's easier/cheaper to use the same columns/carriages and derate the arms, motor, cables, cylinders, and so forth for a lighter model.

Sure, we would love to hear from a lift design engineer. For me, part of the fun of buying and using machines is the process of learning the ins and outs of how and why they work.

That would be great. An experienced perspective on what/how standards are met. Of course, if an actual lift engineer happened by he/she would be a little constrained by marketing and liability, but still interesting nonetheless.

Since I mentioned anchors in my post about the ALI Standard upthread I thought I'd add this comment I made from a another related thread which might be useful to see here (in case there are spectators who did not see it there):

"Speaking very generally here, the tension capability of the anchor will vary with the square root of the concrete psi. (3000psi/2500psi)^1/2 is 0.9 or about 10% derate. Drilling deeper for the anchor will make the pull-out higher by the square of the depth. (5.5 inches/4 inches)^2 or about 1.9 times stronger (although some derate will result depending on how close the anchors are to each other). The thicker concrete doesn't make it stronger, the longer anchors support more tension. Multiply the two effects together, 0.90 x 1.9 = 1.7. Anchors at 2500 psi 5.5 inches deep will have 1.7 times the pull-out strength of an anchor in 3000 psi 4 inches deep. Minus some derate for the closeness of the mounting holes in the lift baseplate."

 

[Diesel Dan]

Yes, I think this is also common. My Benwil lift appeared to have the same column as the next capacity up. Makes sense.

In the case of the worth it "appears" they design the columns for free standing floor plate models that need to be more robust and just carry it over to the clear floor models that have a cross bar brace.

The new, lighter built, lifts being sold today might meet the ALI ratings but I'd rather have a HD lift like the Benwil, mowhawk, worth etc that don't let the vehicle rock as much. While the fasteners and mechanicals will hold you risk the vehicle sliding around on the pads.