Use Aluminum as Friction Bushing? 7

[NuclearNerd]

Hi All

I have an application which involves pre-loading a heavy spring with a cam mechanism operated by a handle, but I'm concerned about the operator losing grip of the handle and having it whip around under the force of the spring.

One way to avoid the problem would be to ensure that there is enough friction at the cam pivot to overcome the spring force. By my calcs, to do this the cam pivot has to have a radius at least as big as 1 / coefficient of friction * eccentric radius. Since my eccentric is 0.3", using the bronze c of f, I need a shaft over 4" in diameter!

Naturally I thought about other materials for the bushings. Aluminum seems to have a very high c of f on steel (over 0.4!). Could I make aluminum bushings in this case, or would the wear behaviour cause me problems such as galling. Any experience is helpful.

 

[zekeman]

I don't know if it was mentioned , but a fixed ratchet would do it ; instead of the operator rotating the arm 180 degrees and reversing, he would go 360 to complete the cycle.
Seems all too obvious.

 

[unclesyd]

I recommended a retching pull handle because of the cost, cheap. But if you want a better selection of ratcheting handles take a look at the Lowell site

http://www.lowellcorp.com/

 

[zekeman]

As noted previously a constant torque cam will reduce to a minimum the the maximum force on the handle.

You can get a constant torque cam derivable from the energy equation as follows:

T*@=k/2*(r^2-r1^2)
where
T= constant torque
r1=initial deflection of spring
k= spring constant
k*r1= preload=100 lb
T is obtained from

T*PI=k/2(rf^2-r1^2)
rf= deflection after 180 of motion
rf=r1+0.75 inches

Solving the energy equation for r
r^2=2/k*T*@+r1^2
r=SQRT(2/k*T*@+r1^2)
Now the cam has a base circle of R0 ,
so the pitch curve of the cam Rc is simply

Rc=R0+r=R0+SQRT(2/k*T*@+r1^2)

You can develop this in a spreadsheet or a layout for reasonable R0 values, the larger the better.The layout will give you the actual cam profile when you superimpose the cam follower on the pitch curve.