Help my dad and I settle a argument

1. Feb 8, 2012

jpowell18

My dad has a car lift in his garage with arms that swing out and extend under the car. He says that when you are lowering the lift without a vehicle on it if you stand on the end of the arm then it will apply more downward force causing the lift to lower faster. I think that it doesn't matter where you stand on the arm. If you weigh 200 pounds you are going to be applying 200 pounds no matter if you stand on the end or on center. I do agree that standing on the end of the arm applies more torque where the arm meets the upright. Am I wrong or right? I've attached a picture of the lift so you can see what I'm talking about. Thanks for the help.

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2. Feb 8, 2012

James Leighe

You are correct, however standing near the edge of the arm applies more stress in the arm itself to hold you up. (this does not effect the downward force however)

3. Feb 8, 2012

Jonnyb42

So if the question is about where on the arm to stand to lower the lift faster, then I agree with you, you apply the same force downward. In fact, standing on the end will give more torque like you said and may squeeze the arm against the vertical part causing more friction and may make it even worse for lowering. I would say you should stand closer to the connected end.

4. Feb 8, 2012

DaveC426913

Well, considering they're designed to hold several tonnes of car out there on the ends of those arms, I'm sayin' torque binding is a problem they've solved.

5. Feb 8, 2012

greentlc

If that is a hydraulic lift then standing on an arm should not increases the speed of lowering. In a hydraulic circuit that is designed to accommodate varying loads, a pressure compensated relief valve is used. This valve ensures that the speed of the actuator is constant under varying loads.

6. Feb 8, 2012

houseii

I would say you should stand closer to the connected end.http://www.bosin.info/g.gif [Broken]

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7. Feb 8, 2012

DaveC426913

Why?

Last edited by a moderator: May 5, 2017
8. Feb 9, 2012

Lsos

They obviously solved it, but still standing at the ends should increase the friction by some non-zero amount. So strictly theoretically speaking, I would agree that standing on the end of the arm would (or could) be slower.

9. Feb 9, 2012

sophiecentaur

It's probably a worm drive with a threaded rod up inside each arm. They are very inefficient (the friction is desirable so you don't need to lock them) so nothing you can do will make much difference. Listen to the tone of the motor to see if you are actually having any effect when you step on and off.
You can both stand out in the garage and argue about that sound too. Then go down the pub and the one who's wrong can but the other one a drink.

10. Feb 9, 2012

Jobrag

Do an experiment, (and post the results).

11. Feb 9, 2012

DaveC426913

It is a big assumption that increased friction translates into slower speed. If the motor and gearbox have a fixed output, then it may well proceed at a fixed rate regardless of any small changes to friction. The friction may simply result in more heat for example.

12. Feb 9, 2012

turbo

My friend and mechanic has one of these lifts in his shop. They don't rely on friction to hold the arms in position - instead there are mechanical stops that ratchet into position as you raise the arms. It wouldn't be safe to work under the vehicle otherwise.

13. Feb 9, 2012

sophiecentaur

Does the car really creep down, once it's been raised? I bet the mechanical stops are only there as belt and braces. I never knew a screw jack come unscrewed whilst I have changed a wheel.

14. Feb 9, 2012

turbo

There are stops in the lift-arms that ratchet into position as the lift is raised.

15. Feb 9, 2012

sophiecentaur

So what do you have to do with the stops to lower it?

16. Feb 9, 2012

turbo

I think he has to raise the lift slightly and release the stops before lowering.

For those looking at the image and imagining lots of binding torque, those arms are not ever in those positions in use. The arms are swung fore and aft on each side to allow the pads to be placed under structurally-sound points under the frame of the vehicle. Those pads are much closer to the posts than you might think, looking at that image. They are not physically closer to the posts, but they are much closer to the center-line of each post.

17. Feb 9, 2012

Lsos

I agree, that's why I specified it "could" be slower. I guess my point is that since there was an argument about this, the dad is less right than the son. It doesn't hurt it to stand on the end of the arm, but from some point of view...perhaps efficiency, perhaps stress, it's not going to be better, but it MIGHT be worse.

18. Feb 9, 2012

mender

My hoist is hydraulic as is the one in the picture. Although the speed range is fairly narrow, there is a noticable difference in lowering rates depending on the weight. Having a person on one of the arms helps speed things up (down) when there is no car.

19. Feb 9, 2012

jpowell18

Thanks for the help everyone. I think I have him convinced I am right. Using some of my daughters toys and a scale, I performed a small scale experiment last night and sent him the pictures. It is a hydraulic lift and standing on the arms does cause it to lower a little faster if there is no car on it.

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20. Feb 9, 2012

DaveC426913

OK.

1]
Your stance was that it would not change. So how can you be right if your experiment showed you to be wrong?

2]
Wait, what? It lowers faster with no car?

So, you're both wrong?

Finally, and most egregious:
3]
How does a completely different device that uses a completely different mechanism say anything at all about the real-world behavior of the full-scale hydraulic lift?

21. Feb 9, 2012

jpowell18

The original argument is if standing on the end of the arm would apply more down force than standing on the arm closest to the upright. In the first picture the kitchen scale shows 100 grams with the weights directly on top of the scale. The second picture shows the same 100 grams with the weights on the end of each wood piece. Seems to me the scale shows a down force of 100 grams no matter where the weight is positioned on the wood pieces. Scale that up to the car lift and and no matter where the person is standing on the arm the same amount of down force is being applied.

1] I was responding to a previous posters question asking if weight on the lift caused it to lower faster.

2] Probably could have been worded a little better, Once again I was saying that lowering the lift without the weight of the car is slow and having a person stand on it does speed it up. Doesn't matter where the person stands. The extra weight just helps lower it faster.

3] Because we are not talking about the behavior of the lift. We are discussing the amount of down force being applied if standing on different areas of the arm.

Not sure what is hard to understand if you had read this thread from the beginning.

22. Feb 9, 2012

DaveC426913

Indeed, I have been following from the beginning, and there are some ... problems with your explanations.

Well of course there's no difference!

The issue here is not whether there's more force, but whether the torque has any effect on it.

(At least, that's the issue most of us were discussing. It is trivial that the weight of the person does not change, the only variable was the torque.)

Sorry, we'd moved past that by post 3.

OK, we knew that. We were looking at a different issue. Very soon after your first post, it became apparent that he only mitigating factor was the torque.