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Torsion spring design issues in a gripping device

Torsion spring design issues in a gripping device

Torsion spring design issues in a gripping device


I am an industrial designer and have designed a miniaturized gripping device that contains a torsion spring in a chamber to force the handles of the device into an open position.  The device originally opened 30 degrees, however we needed to shorten the device, causing the new opening size to be around 50 degrees.  The chamber for the spring is very small 2.5 mm deep x 6.3 mm diameter.  There is a 2.6 mm dia pin that runs through the center to create the axis of rotation.
The current torsion spring size is 5.5 mm od, 3.9 mm id, with 3mm legs normal to the coil. wire size is .7mm with 3 coils
Basically the problem is our original spring is designed for a 30 degree opening, and we now need to open to at least 50 degrees.  We could increase the diameter of the spring, but it would cause us to need to lengthen the tool.  We could also add coils, but then our thickness increases too much.
The big question is can we use a different material to get more flex out of our spring?  Or maybe there is an option we are overlooking.
I have redesigned the tool already using a flat spring, however my client/inventor insists on a torsion spring hence my dilemma.
Thanks for your insights.
Sean K.

RE: Torsion spring design issues in a gripping device

When you say normal to the coil do you mean in the spring axis direction or radially to the coil?

Do you have any torque requirements at the 50 degree deflection?

Is the spring preloaded with some load/deflection in the zero deflection of the gripping arms? and if so how much?

I am confident that a new torsion spring can be designed in the given space but answers to the previous questions are needed.


RE: Torsion spring design issues in a gripping device

Cold headers started out with the flat spring assisted grippers, next some with torsion springs, following designs used coiled springs in compression or tension, later designs are rack&pinion or powered grippers.  

The Star metal link is probably the best data available to show all the different type of grips.  That company copies all manufactures of presses for transfer/gripper tooling.  Thay have hundreds of designs.  Headers are high speed precise machines, very capable.



Did you contact your spring manufacture?  They can probably assist with good knowledge.  But, sorry to say, my luck with getting a spring company's technical assistance hasn't been very good.

Feeling frisky.........

RE: Torsion spring design issues in a gripping device

The direction of the legs are normal to the axis, not parralel with it.

I can post some images online if it will be helpful to visualize the spring, and the context in which it is being designed for, as my terminology may be a little off.

In terms of the load, in the closed position I am looking for about 8 lbs (3.6 kg) resistance.  In the fully opened position the spring should be preloaded with .5 lb (.23 kg) pressure.

Right now the tool is designed to increase the body diameter.  This could change easily enough if it will benefit the amount we can open the tool, or it may increase the life of the spring to compress the body.




RE: Torsion spring design issues in a gripping device

From analyzing your current spring the torque at 30 degrees deflection is only 2.43 kgf*mm. Therefore the force on the legs is approximately 2.43/4.8=0.5 kgf (4.8 is the spring mean radius).

If you are looking to receive 8 lbs (3.6 kg) resistance for a spring this size I am sorry to disappoint you. But using a different materials you can get a 50 degree deflection or even more but you need to consider spring number of deflection cycle that can spring can have before it will fail due to metal fatigue fatigue.

Can you post images online?

RE: Torsion spring design issues in a gripping device

Yes, that is correct.  The current deflection at 30 degrees is .5 lbs.   My wish for 8 lbs was actually a mistake.  I was trying to judge the optimal resistance by estimating the 'feel' of a spring loaded product I have with me.
I do have a prototype of the current product with a 30 degree spring and it is pretty good.  Now that I see the torque is .5 lbs, I can make a accurate judgement.  But, as I mentioned we have had to increase the opening to 45 degrees as seen in the link below, and another instance at 60 degrees.  So basically if we can increase our opening, with the same sized spring, and increase the load just a bit to say .6lb or .75 lb resistance that would be optimal.  

Thanks again for your insights.
Sean K.

RE: Torsion spring design issues in a gripping device

You did not specify what is the desired life cycles from the spring. This will dictate the load and deflection as long as you can not change the spring dimensions.

RE: Torsion spring design issues in a gripping device

We would like our product to have a lifetime warranty.  Meaning that the spring will never wear out.  I know this may be ideological, and if we had to put a number on it I think it would be around 300000 or 400000 uses.  The other factor that may be an issue is the fact that in the stored mode, the spring is compressed.  essentially this means it is always under load.  Does this effect the amount of lifecycles?

This is really helpfull for me to see the spring from all angles, and I certainly appreciate your insights and questions.


Sean K.

RE: Torsion spring design issues in a gripping device

How much the spring is compressed (deflected) in the stored position?

10,000,000 torque cycles are considered unlimited lifetime in machine design.

If a spring is compressed without release for a long time the issue of relaxation has to be checked. However, it does not have an influence on the fatigue phenomena. Fatigue is a failure due to repeated loading between two points of load.

One more thing, torsion springs are springs that create torque and not force. To correctly define a torsion spring you must specify the torque needed to deflect the spring a known amount of angle with respect to it axis. Torque units are force x length such as lb x inch.

Till now I am not sure what the spring has to endure.
Can you define two points of deflections from free position such as angle1=? and angle2=? and respectively Torque1=? and Torque2=?

Only with these two points information one can design a spring and may find a material that will allow such requirements (including life cycle).


RE: Torsion spring design issues in a gripping device

I've changed the design a little in terms of od, id, and the spring compresses rather than extending to get the load required.

See the attached link.  It provides all the dimensions, as well as the 2 positions of the leg, compressed (the stored position) and relaxed.


My problem is that I don't exactly understand how to calculate the load.  I had a link before that showed a nice schematic of the equation, and how it relates to the spring, but I can't seem to find it now.  I know at the 60 degree compressed state, I would like to see 1 lbs of force on the legs achieved.  Then around .5 lbs of force on the legs at a 30 degree opening. Hopefully this helps.

The spring currently has 830 degrees of coil in the shown schematic.

Perhaps you can help by explaining to me how to calculate load, then I can provide exactly what you asked me for in your last posting.


Sean K.

RE: Torsion spring design issues in a gripping device

Hello Again,

So based onm this drawing:


If the Spring deflection is 60 degrees, the spring moment is .246", and the force exerted on the spring is 1 lb, then the spring constant is .0041

If the spring deflection is 45 degrees, the spring moment is still .246", and the force exerted is proportional at .75 lbs, then the spring constant is still .0041.

Hopefully these are the two points of reference you requested.  Is the spring constant the torque?  Not sure how the constant relates to the design.  This is the formula I used.


Sean K.

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