Double torsion spring vs. two torsion springs (left and right hand) 2

[kungfumel]

I am designing a mechanism with a torsion spring on each side of a base that work together to hold an arm down. Each spring is warped around a 0.7" boss. The arm is 17" long. I am trying to get 10-lbs of force at the tip of the arm when it is opened out 135 degrees. I have attached a sketch of the set-up.

I was thinking of using two torsion springs (a left and right hand) and each spring would be 4mm wire diameter with 5 coils.

Will this give me the appropriate force of 10-lbs at the top of the arm?

If I used two springs instead of a double torsion spring will it act the same?

How does having 2 springs effect the force? Do the forces from each spring add together?

Are there enough coils?

 

[ShaggyPE]

You should really talk to a spring vendor to work out the details of your spring design. Here is a good one:

http://www.centuryspring.com/

Alternatively, here a link to an online calculator.

http://www.engineersedge.com/spring_torsion_calc.htm

-Dustin
Professional Engineer
Pretty good with SolidWorks

 

[KENAT]

The equations for torsion springs should be available in a good reference book. I think we actually used the relevant British Standard when we were designing some 10+ years ago. I really recommend looking at the actual equation as it will give you some feel for how the variables act together which can influence your design beyond just the spring.

The double torsion spring will act similarly to 2 single torsion springs, and yes the forces should simply add. Double torsion springs can be easier to mount/install/assemble etc. than 2 separate springs. It will of course depend on your application but I strongly suggest looking at the double torsion spring.

Posting guidelines faq731-376

http://eng-tips.com/market.cfm?

(probably not aimed specifically at you)
What is Engineering anyway: faq1088-1484​

 

[israelkk]

kungfumel

Where is the sketch?

 

[israelkk]

17" arm? This is more a beam than a torsion spring. The arm deflection will be huge due to the the bending of the arm. the arm will not stay straight therefore, you can not ask for rotation angle of the arm. Torsion spring is designed for torque not for force. If the force is at the tip of the arm is 10 lb and the arm is 17" then the torque is 170 lb-in.

 

[kungfumel]

Thanks for the ideas so far!

It looks like the sketch didn't attach the first time, I am attempting to reattach it.

 

[asimpson]

You could use C-flex bearings/torsion springs. They act as pivot bearings with limited rotation in either direction and spring restoring tosion.

http://www.c-flex.com/stiffnessproperties.html

 

[israelkk]

asimpson

How C-Flex can do 135 degrees?

 

[israelkk]

5 coils of 4 mm wire diameter can not handle 135 degree deflection and torque of 85 lbf-in (for each of the two springs).

 

[asimpson]

multiple C-flex would increase deflection at expense of stiffness.

 

[zekeman]

Forgive me, I looked at your large mousetrap and roughed out a design based on
tables in Machinery's Handbook as follows

For a working stress of 140,000 psi with an ID=0.8125", you need a wire size of 0.2437 inches with a deflection per coil of 8.346 degrees and an allowed torque of 199 lb-in (determined by the wire diameter); you need 170 lb-in)
So you need a total of about
150/8.346= 18 coils
or 9 coils on either side

assuming 15 degrees preload.

You can reduce the torque to 170 by increasing the number of coils or increasing the ID of the spring ; for the former, I get
199/170*18=21 coils ; the stress would also decrease in the same proportion.

A useful design formula
ND= E*d^4 *defl/(4000*T)
allows you to manipulate N and D to get the best configuration.
N= active coils
d= wire dia
E= modulus of elasticity
T torsion required
D mean spring diameter =ID+d
defl total deflection including preload

Now you can go to your friendly spring coil vendor with some idea of what you need.

 

[Cockroach]

Very nice work, ZekeMan, star for you!

Regards,
Cockroach