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.

 

[ivymike]

well, aluminum pistons seem to do well enough w/fully floating steel pins (lubed/w hot oil bath).

 

[EngineerTex]

I've pulled out my hair trying to get predictable and repeatable performance out of friction materials in dynamic applications. Every time the two parts rub, you are changing the mating surfaces. You may be burnishing the two in one application where it gets smoother and smoother so that it loses its grip, while another application may have dust, dirt or other contaminants which act to increase the friction between the parts. It's a crapshoot.

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.

 

[hydtools]

The friction would add to the effort required by the operator.
I'd use a ratchet or one-way Sprag clutch on the crank axle.

Ted

 

[NuclearNerd]

The problem with a ratchet is that the spring needs to be both loaded and unloaded by the handle, so any ratchet would have to have a releasable catch - that gets pretty complicated! I was really hoping to do something simple like change the bushing material / diameter, but as Tex points out - my solution has its weaknesses.

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

 

[desertfox]

Hi NuclearNerd

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

 

[NuclearNerd]

No way to load the spring without loading the handle - I don't want to bring in any external power sources like compressed air. I've attached a sketch done in PowerPointCad

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.

 

[hydtools]

Use a steering wheel sort of arrangement. If the operator loses grip, there is only a spinning wheel and not a single-ended lever.

Ted

 

[drawoh]

NuclearNerd,

Your application is the precise description of a damper. Could you use a hydraulic or pneumatic one?

JHG

 

[kingnero]

Would a lever work for you application?
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.

 

[CH5OH]

if the operator has grip on the handle, the chance of getting smacked by the handle is the smallest.
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.

 

[ornerynorsk]

Contrarily, I would use a bearing as the od of the eccentric and just make sure you have a handle of sufficient length with an elastomeric covering for good grip. You could use a gas damper if you absolutely had to in order to idiot-proof it.

 

[desertfox]

Hi NuclearNerd

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

 

[NuclearNerd]

@desertfox: The handle pivots the cam which swings around the top before pushing on the spring. At the half way point, the line of action would be 0.375" above the pivot point, giving a moment of about 37 in*lb, which is a fair bit for a handle.

 

[desertfox]

hi NuclearNerd

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

 

[unclesyd]

I'll go with desterfox put the handle on the cam.

If this is not feasible get hold of a retching pull handle from someone like Harbor Freight. A built in one way reversible clutch.

 

[NuclearNerd]

Thanks for the sketch DesertFox. I'm using a circular profile for my cam (it's actually a roller), which necessarily puts the line of action eccentric from the pivot axis. A cardioid (or some such) profile would certainly reduce the moment (and make the moment more constant over the travel range) - I'll look into that.

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.

 

[desertfox]

hi NuclearNerd

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

 

[NuclearNerd]

Hi desertfox

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!

 

[desertfox]

hi NuclearNerd

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://www.ul.ie/~kirwanp/whatisacamandfollowersyste.htm

desertfox