Our scissor problem pt1

[whytheheckme]

Just uploading pictures..

This is the robot not under stress

[whytheheckme]

More pics of the bot not under stress

[whytheheckme]

Here it is under stress. Note how high the scissor has been lifted. Any more height after this point causes the center brackets supporting the center bar supporting the motor to bend, causing the center bar to fall. If we perminatly attach these as opposed to clamping them, it bends the center of the chassis down about 2 inches, something we did not want to do again...

[whytheheckme]

Finally, some pictures of the motor mount.

[whytheheckme]

This is a picture of the support piece that was bent by the downward force of the scissors as the motor was trying to lift it.

[Andrew Blair]

Because in the photos that you call "under load", there is no actual load other than the scissors being lifted, I am assuming that the issue is not simply too much loading, but a problem in the actual design and implementation of the lift. Correct?

As to your issue- As you probably have already figured out, there is a bind or mechanical cinch point which is causing the force needed to raise the scissors to approach that which your mount is failing under. For this type of lift, which I assume is being used only for tubes, your bent mount should be plenty strong enough, when it's all resolved.

My thoughts are that you are not allowing the lead screw to move the proper amount laterally, based on the look of the lift. It appears that you've anchored one side of the lift, and are allowing the other side to move on rollers. It also looks like there is provision for angular change at the point that the screw connects to the loft, but your motor is tied fairly statically to the bar. It cannot rotate to account for angular change, while the connection point is shifting laterally. So , you bind at the point where the motor starts to want to rotate, but cannot. Or maybe I'm looking at your lift all wrong.

[ChrisH]

Originally Posted by whytheheckme

Here it is under stress. Note how high the scissor has been lifted. Any more height after this point causes the center brackets supporting the center bar supporting the motor to bend, causing the center bar to fall. If we perminatly attach these as opposed to clamping them, it bends the center of the chassis down about 2 inches, something we did not want to do again...

My initial thought was that you are experiencing the classic "low angle" problem so common on scissor lifts. This occurs when you are trying to extend the lift but the force is appied at a low angle relative to the motion. This leads to very high forces.

Taking a closer look at the photos it became clear this is not the case. Quite the opposite, you are applying force at or close to 90 degrees to the direction of the structural members, so this should be an optimal situation.

From you pictures it is not clear whether or not one end of the scissors members are pinned as described by Andrew Blair. If either end is pinned then you will start having to fail a joint or other structure to get movement. So make sure the end joints can move in and out freely.

Another thing to remember is that Work = Force*distance. The scissors lift is acting like a distance multiplier. You move your bottom pins a small amount vertically and get a larger vertical movement from the upper stages. This means that the forces go the other way. A small force at the top gets greatly magnified at the bottom of the scissors.

Two other things to consider:

1) Does the window motor have enough power to do the job you want it to do? They only put out 22W. Can you lift the entire lift into extended position on just 22W? 22W is about 17 ft lbs/sec. Based on the pictures it looks like the lift weighs 12-15 lbs, which means it will take about 1sec to lift it an average distance of 1 foot, assuming close to 100% efficiency. If your gearing doesn't take this into account you will continue to have problems.

2) You can more than double the strength of the piece you bent but mounting it with the skinny side up rather than fat side up. But the end suports look a little flimsy. Just turning the bar will probably only move the failure point.

[Dick Linn]

Pop off the acme nut, lift the mechanism manually and see if you can tell where it's binding.

[dtengineering]

We made a scissors lift for our very first FIRST 'bot four years ago. I think you are experiencing something that we experienced on that design....

Judging from the photos the screw is able to lift the bottom stage of the lift almost perfectly... but the top stages stubbornly remain locked in place horizontally. When a member, pinned at both ends, approaches the horizontal, it can only convey horizontal force. You need the upper members of your structure to be able to convey vertical force... but since they are bascially horizontal they cannot.

To confirm this, try supporting the top of your lift (hold it up) so that the members are at least 30 degrees from the horizontal. Now try running your screw up and down with the motor. I suspect it will work just fine. Now try lowering the lift all the way down... is there a point where the upper stages "collapse" while the lower stages, supported by the screw, do not?

If this is the case then there is nothing wrong with the lift itself. (In fact, it looks like a very nicely designed scissors lift). You problem lies with gravity and physics.

Some things we discovered with our scissors lift: don't collapse it ALL the way down... this robs the members of any ability to convey vertical force. Secondly, to aid with the lift, try wrapping bungee cords around the "spacer" pieces that keep the two sides of the lift running parallel (I can NEVER spell parralell correctly...) so that the lift is pulling itself together.

Hmm... that doesn't make a lot of sense... try this lovely piece of ASCII art...
.....\/
...../\
.../....\
./........\
OBungeeO
.\......../
...\..../
.....\/
...../\

That will take a lot of the load off the motor, if nothing else.

Play around with it... scissors lifts are cool... and so are the mecanums.

Jason

[Al Skierkiewicz]

Jacob,
I go along with Chris on his assesment and the others. You have a tremendous amount of force that you are trying to overcome. As you start to raise the scissors, you are encountering some rather exceptional friction in the horizontal direction and well as trying to raise the weight of the assembly. That in of itself is bad enough, but it also appears that you trying to lift a motor, gearbox and shaft as well. That throws all the forces off balance and multiplies the work that the lead screw and motor must perform. When you try this again, take the motor and other parts off the top section and see if balancing helps. As pointed out, try lifting a few inches by hand and see if the motor can take over from there.
Lastly, the joints in a scisor lift must stay in position and have low friction. As you can see in one of your stress photos, the bottom of the scissor has started to move but the top layers have not yet moved at all. This is the free play in your joints adding up. When the joints have little free play, all layers should move about the same. It is hard to tell from here, but a washer between bars helps reduce the friction of the joint. I would also recommend you use a lock nut (nylon insert) instead of the dual nuts. That will give you some additional control over the friction of each joint.