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Up and Down Do Not Generate the Same Load Curve

Up and Down Do Not Generate the Same Load Curve

(OP)
We have a problem understanding the functional hysteresis in the device shown in the image. The force applied the lower link in the picture is from a spring pack capable of producing 9 to 45 lbs load. The attached graph shows the upward movement of the device's handle requires more force than the downward movement of the handle. The handle is connected to the lower link via a gear train. The gears are supported by 303 precision ground shaft supported by IGUS J Glide journal bearings. An over center spring mechanism holds the handle in the up or down position. We have another variation of this device and get the same hysteresis in the curve.

We mount a load cell on the handle and another load cell between the spring pack and the lower link. The load from the spring pack is fairly constant up and down.

What is unexplained is the up forces are much higher than the down forces. Can any one offer a suggestion why the curves are not the same for the upstroke and down stroke?



This image is a simplified section view of the device.



This curve is without the over center mechanism installed.

RE: Up and Down Do Not Generate the Same Load Curve

Trying to figure out what you are doing... so when the handle is down both the cable and spring apply torque in the same direction on the lower gear while when the handle is up the spring moves "above" the lower gear pivot point so that the spring and cable apply torques in the opposite direction on the lower gear?

RE: Up and Down Do Not Generate the Same Load Curve

When the hadle is moving upward its working against gravity so a force is required to balance its mass, the handle on the way down uses its mass to assist the movement down.
Not sure it's the complete answer but its a start.

RE: Up and Down Do Not Generate the Same Load Curve

A question, when the handle is lifted I assume the spring starts to get compressed and as it does so the load increases,whereas when its lowered the spring helps the downward movement.
Perhaps you can give us more information so we can help you.

RE: Up and Down Do Not Generate the Same Load Curve

(OP)
Let me add more detail to this thread:

The cable load in the image starts at 9 lb at the beginning of the stroke and applies 45 lb at the end of the stroke. The increase is linear.

The spring in the image is used to push the handle down when released. There is some friction in the cable that resists the downward movement of the handle at the end of the down stroke.

The spring is an overcenter mechanism. At the end of the UP stroke, the spring has crossed its center point and assists in the upward movement of the handle by lowering the required force to lift the handle at the end of travel.

The problem is that as the graph shows in the other image file there is hysteresis in the system. The up and down stroke forces as measured at the handle are not the same. The effect of gravity on the handle weight is negligible. The cable force is measured during the up and down stoke and it's input is linear from 9 to 45 and back to 9 lbs.

The question is what could be causing the hysteresis?>?????? bugeyed

RE: Up and Down Do Not Generate the Same Load Curve


How long is the cable, and how is it constructed? Could there be elastic strain in it that is being generated on the way up that is released on the way down?

RE: Up and Down Do Not Generate the Same Load Curve

How is the force being measured? Does the measured force remain at a right angle to the handle? Or is it always vertical?

Does the load on the cable eventually move up/down vertically? If yes, what are the accelerations involved in moving the load? The upward acceleration adds to the gravitational acceleration, while the downward acceleration subtracts from the gravitational acceleration.

RE: Up and Down Do Not Generate the Same Load Curve



I didn't use the exact right words to describe what I meant, but look at the "Elastic Hysteresis" section under the "Hysteresis in Mechanics" heading in the following link. If the cable is not rigid, it could be behaving like a rubber band.

http://en.wikipedia.org/wiki/Hysteresis

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