Quote:
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.