Mechanics of a lifting arm

[emarthinsen]

Hello-

This might be a basic question, but here goes. I'm trying to design a counterweighted lifting arm for an adjustable work surface. I attached a picture that shows the basics of what I am looking for. Essentially, I'm looking for a mechanism that will allow the work surface to be raised or lowered. When it is raised or lowered, it should move only in the y direction and not in the x direction. The counterweight is needed so that someone can raise and lower the surface by hand, without needing any assistance. Lastly, this is for public consumption, so style, reliability, and simplicity matters (not that private consumers don't care about such things). Does anyone have any ideas on how to pull this off? Is there any prior art that I can study to get a better sense of how others have approached this problem?

Regards,
Eric

 

[drawoh]

emarthinsen,

Your device looks like a scissor lift. I think most scissor lifts use worm gears. High ratio worm gears cannot be back driven. This is good.

If your mechanism is not self locking, you are doing to have to design a simple, easy to use, and powerful lock. This may be surprisingly difficult.

--
JHG

 

[rb1957]

if you have a hinge at the 90deg angle in the "lifting arm" so the angle between the two arms can change, and have a vertical track (so that the table is constrained to move in the vertical direction only) then i think you've have what you want. i'm not so sure about the efficiency of the counter-weight (but what do i know ?).

 

[zekeman]

Look up a window sash mechanism with counterweights over 2 pulleys.
Your sketch will not do it, since the counterbalance effect is too variable.

 

[drawoh]

zekeman,

How about all the stuff that may or may not be left on the table prior to lifting or lowering?

I don't think this is a good application for counterweights.

--
JHG

 

[zekeman]

"zekeman,

How about all the stuff that may or may not be left on the table prior to lifting or lowering?"

--------------------------------------------

I don't think this is a good application for counterweights."

True, but if the guy likes counterweights and the table weight is the predominant force, then let him decide.

My own vote is a scissor jack which, BTW only needs a jackscrew and I never saw an automobile lifted with it that didn't self lock.

 

[emarthinsen]

Thanks gang. Good info. I'm hoping to avoid a scissor lift or anything with gears or drives. Ideally, something more akin to the arm on a desk lamp. I know that doesn't apply perfectly here, but that was what I was shooting for. Maybe is there a way to have parallel arms (coming back to the desk lamp here) and as they are raised an lowered, they change the position of the connector at the "elbow" of the lifting arm such that it moves other part of the arm appropriately? Thinking out loud here. Perhaps what I'm hoping will work is actually not possible. Might be time to break out some cardboard and bolts and see what happens.

 

[Cockroach]

Slot the top end of the lifting arm below the table and place that on a roller. As the arm moves in a circular arc around the pin, the table will move up when constrained in the axial direction.

Very simple design. I agree with DrawOH, pretty much a modified scissor lift. I doubt there is significant patent protection on this, lots of heavy equipment use this mechanism for platform motion. I would look at heavy equipment supply catalogs, maybe platforms on the back of trucks that need vertical lift of goods off the bed to the ground. Some forklifts use this mechanism.

Good luck with your project.

Regards,
Cockroach

 

[1gibson]

One idea - use spring(s) instead of the counterweight, allow adjustment of spring tension (threaded rod with hand wheel, attached to end plate of spring) to adjust spring tension to match weight of workpiece / vertical position of table. Add significant damping, and mechanical lock on vertical position.

Less weight, more versatile, and allows you to keep your same basic design. Just remove conterweight, attach spring(s) to same location, other end goes to adjustable plate.

If you double up and make two lever arms, that can serve to constrain motion in horizontal planes. At this point I have more or less described a scissor lift but the two arms don't need to be mechanically attached at a pivot point. Tried to still retain the basic concept of your original design (minus the counterweight.)

But, I also vote scissor lift.

 

[rb1957]

why avoid a scissor lift ?

do you want continuous adjustment or a set number of rest postions ?

do you want to raise and lower, or just lower (with one hnad) ? a gas strut could easily lower and lock.

will there be two lifts with the load between, or one (with the load off-set) ?

 

[drawoh]

1gibson,

Spring force will not be very constant with height unless you work out a tricky mechanism. A gas strut could be designed to be constant, but an off the shelf gas struct is basically another spring.

--
JHG

 

[1gibson]

drawoh, agreed, but neither would a counterweight on a lever. I didn't come up with the original design , and remember I still voted scissor lift The "tricky mechanism" would be adjusting the spring length based on desired vertical position. Not that tricky, all things considered. Same could apply to a gas strut, which is definitely a better solution because it has built in damping, and no pinch points like a coil spring would.

 

[hydtools]

Another vote for the scissor lift.
Desk lamp depends on adjustable friction in the joints to help hold a position, not just the springs. Those I have used barely support themselves, let alone added and removed weight.

Ted

 

[hydtools]

Consider a barber chair type lift.

Ted

 

[FeX32]

Springs or counter weights will in fact work given one condition. The table's mass does not change much.
Essentially, design the system in such a way that its potential energy does not change with the range of motion desired. This is not a really hard problem, but it is clever. I have done some theoretical design with this principle before that worked.
The advantage of this is that once moved the table will in fact stay put (just like a desk lamp..)

Scissor jack is more industrial and can be used with varying weights. Use it, if you need variable weight movement. Use a clever design if you don't.

Cheers,


Fe (IronX32)

 

[zekeman]

Take the OP design and add a circular cam ( wheel) at the end of rod so that the distance from the pivot is the same as the counterweight distance to pivot which now puts it in force balance. As it lifts the cam rolls on the table top and is in force equilibrium, except for friction.
Use two, in a scissor arrangement to get parallelism of the table top, keeping the CM aligned with the common pivot.

 

[FeX32]

Interesting thought zekeman.
Although if I understand you correctly, the potential energy of the entire system still changes with a movement of the table. ie. the table will not stay in one place unless you somehow cancel out the change in potential energy of the table mass. (with a mass larger than that of the table, or spring arrangement) Of course, with the diagram the OP has given we can assume the table has no mass
I may not have understood your description however.


Fe (IronX32)

 

[zekeman]

"Interesting thought zekeman.
Although if I understand you correctly, the potential energy of the entire system still changes with a movement of the table. ie. the table will not stay in one place unless you somehow cancel out the change in potential energy of the table mass. (with a mass larger than that of the table, or spring arrangement) Of course, with the diagram the OP has given we can assume the table has no mass pipe
I may not have understood your description however. "


No, the table has weight= CW weight and the change in PE is now zero, since the table movement upwards equals the CW vertical movement downwards for this system.; however, if you have room, you could reduce the CW weight with a proportion increase in mechanical advantage by linkage ratios.

 

[FeX32]

Gottcha. Yes, I agree with you in that case.
The OP could add a mild torsional viscous damper to the design to reduce inertial fluctuations.




Fe (IronX32)