Use Aluminum as Friction Bushing?
Use Aluminum as Friction Bushing?
(OP)
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.
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.





RE: Use Aluminum as Friction Bushing?
RE: Use Aluminum as Friction Bushing?
I would vote for redesigning the part.
Try:
1) Replacing your rotating handle with a sheave with a length of wire rope on it. Affix a handle on the end of the wire rope so that you can prevent the problem of a handle coming back to nail someone in the face.
2) Add a ratchet to it which can be released under load.
3) Add a brake -- yes, it would be a frictional brake, but not one which relies on repeatable dynamic coefficient of friction. And you would design it such that it is only released when the operator's hand is on the handle.
And the other thing is that your aluminum friction bushing might work absolutely perfectly, until the maintenance technician comes along with a can of WD-40.
Engineering is not the science behind building. It is the science behind not building.
RE: Use Aluminum as Friction Bushing?
I'd use a ratchet or one-way Sprag clutch on the crank axle.
Ted
RE: Use Aluminum as Friction Bushing?
If people want to brainstorm more generally here are the rest of the design details: The spring is preloaded with 100 lbf of force. I need a mechanism to compress the spring further (by 0.75") with a quick turn of a handle. The cam method works like a charm, except for the forearm-breaking part :)
RE: Use Aluminum as Friction Bushing?
Can you post a picture of your mechanism? there is a way to make the handle operation load and unload the spring without transferring load to the handle.
desertfox
RE: Use Aluminum as Friction Bushing?
After a night sleeping on it, I think a good solution might be to insert a commercial rotary damper around the handle shaft, or wrap the spring around a damper (I need a through-shaft though. Maybe I'll make a damper using an air cylinder with an exhaust valve). Dampers get rid of the flying handle problem without adding significantly to operator effort the way friction bushings would.
RE: Use Aluminum as Friction Bushing?
Ted
RE: Use Aluminum as Friction Bushing?
Your application is the precise description of a damper. Could you use a hydraulic or pneumatic one?
RE: Use Aluminum as Friction Bushing?
pivot point a few inches above the plunjer, hande pointing downwards.
You need a minimum effort this way, and the spring can't launch the handle forward when the pivot point is close enough.
Or like a handbrake lever, where you have to pull. That way, if the spring is pushing, the lever would launch towards the wall or apparatus, hence away from the operator.
RE: Use Aluminum as Friction Bushing?
So you can provide a simple clutch (pin/hole setup), which engages when operator has a firm grip (like a handbrake on a bycycle).The handle will then pivot downwards on gravity,
when the clutch is disengaged.
RE: Use Aluminum as Friction Bushing?
RE: Use Aluminum as Friction Bushing?
Looking at your diagram it appears that the spring force line of action is through the pivot point of the cam, so if that is correct how can the spring force drive the cam backward as there is no moment and therefore no risk of the handle being driven backward if the operator lets go, or have I missed something.
desertfox
RE: Use Aluminum as Friction Bushing?
RE: Use Aluminum as Friction Bushing?
This is what I was thinking see the uploaded file.
To me the only force the operator overcomes is friction, I can see the handle pivots the cam, but what I am talking about is the rotational centre of the cam not what drives it.
desertfox
RE: Use Aluminum as Friction Bushing?
If this is not feasible get hold of a retching pull handle from someone like Harbor Freight. A built in one way reversible clutch.
RE: Use Aluminum as Friction Bushing?
Note that there will always be a little bit of a moment on the cam - you can't climb a hill if the slope is always zero! Or looking at it another way, you cant add energy to the spring without doing work, and you can't do work unless there is a force on the handle.
RE: Use Aluminum as Friction Bushing?
You will have to put energy into the system I agree, however if you have a system as per my sketch the only force you need to overcome is friction, but the friction increases as you compress the spring, why not put the roller on the end of the spring shaft and let the cam, push against the roller to move the plunger forward.
desertfox
RE: Use Aluminum as Friction Bushing?
I agree that putting a roller on the spring shaft and using a better cam profile would reduce the load on the handle. I just sketched it, and the lowest offset I can achieve between the force vector and the pivot axis is about 0.25" (for a 0.75 inch total travel over 180 degree rotation). That still leaves 25 in*lbf of torque to deal with on the handle (minus friction).
Your sketch "appears" to have no moment, but it's deceiving. The point where the cam and the plunger contacts will always be a little above the pivot axis - otherwise the cam slope would be zero and you wouldn't have a cam, you'd have a cylinder!
RE: Use Aluminum as Friction Bushing?
Any chance you can post the cam and force vectors please?
In the meantime have a look at these sites
http://www.technologystudent.com/cams/cam1.htm
http:/
desertfox
RE: Use Aluminum as Friction Bushing?
Seems all too obvious.
RE: Use Aluminum as Friction Bushing?
http://www.lowellcorp.com/
RE: Use Aluminum as Friction Bushing?
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.