Hi. I can't find answers searching for this subject.
I have a mechanical design where I have two clamped rotating discs that are rotated manually, so this is about ergonomics to some degree. I am trying to achieve that 'hydraulic' feeling of the volume knob on a stereo but I am limited to sliding surfaces; actual hydraulic design (fluid flowing through an orifice) is not feasible for me.
I have tried numerous combinations of lubricants and materials and although it feels oily-smooth when I rotate it, 24 hours later I have to overcome an unacceptable of static friction to get it started, then it rotates smoothly again even if I stop. I have not timed how long it takes to seize up but it is not while I am twisting the joint back and forth.

My best result for the oily-feel is using a heavy silicone grease on the either acetal or ptfe disks. The other side of the grease is a fairly shiny smooth anodized surface. It is a sandwich around a static base plate so there are two slip-disks clamping in opposition to each other on a common wall. I will try to illustrate the stack-up from one end to the other:
-thick metal disk (1/4" x 3" diameter), this spins with the center bolt, which clamps to nothing except the far end of the stack, another stiff metal wall.
-super-glue
-acetal or ptfe disk (15 mil) (etched on one side)
-very thick silicone grease (Molykote)
-shiny thick metal wall (~1/4") that is anodized on both sides
-very thick silicone grease (Molykote)
-acetal or ptfe disk (15 mil) (etched one one side)
-super-glue
-heavy washer (30 mil steel). This is to distribute the o-ring force evenly across the plastic disks
-buna o-ring for the spring force (it's a size dash 214 for what it's worth)
-solid backing for the o-ring

A large bolt running through the stack to clamp it together.
The whole stack is hard-clamped to a fixed gap-width using a stack of shim washers around the center bolt. The size and durometer of the o-ring determines the clamping force. I change the shim thicknesses to adjust the clamping force. I have a nice medium stiffness when rotating and it has a good glide. But it seizes up after sitting for a while. I am assuming that the grease is being squeezed out somehow and is being re-distributed after overcoming the initial sticking, but that may be a completely wrong assumption.

I seemed to have solved this problem once but I wasn't able to determine what has changed, something subtle I'm sure. Am I designing everything right but something is not right in the build?
Any ideas or references to other sites would be greatly appreciated. There must be a developed art to doing this kind of controlled action.
thanks
Matt

 

What you are seeing here is the difference between dry friction and "sticktion" and viscous shear.

Dry friction is mostly independent of velocity, however, "sticktion" (greater friction when velocity is zero) is a common phenomenon. Some materials (e.g. teflon) have less sticktion than others, but all exhibit it to some degree.

The "oily" feel you describe is the force due to viscous shear of a fluid between two non contacting surfaces. Here, the resisting force is proportional to the velocity, and zero at zero velocity. So naturally it will "feel" very different.

To maintain the viscous feel, you need to maintain space between the surfaces, rather than having springs push the fluid out so that you then get contact between the surfaces (and thus sticktion).

 

You might try adding a UHMW polyethylene washer to the stack. These do not require lubricant. You can adjust the feel by how hard you clamp the stack.

High temperatures would present difficulty by causing the washer to lose thickness (compression set). But it the temperatures are not so high it might be a good solution.

 

Dana, your term of "sticktion"("greater friction when velocity is zero) I think is incorrectly used. In physics we call it "static and dynamic friction" As a class we measured the static and dynamic friction in newtons on various materials. Also, there should be charts online what materials exhibit static and dynamic friction. BTW, I remember looking at the chart. The one material that had the lowest static and dynamic friction was joints in the body.

 

'Sticktion' occurs when the friction between two or more interacting components reaches an infinite degree - an easily understandable example of this would be to imagine a hollow oval profiled cylinder which is driven by a smaller solid oval cylinder that fits within the oval profile of the larger cylinder. When sufficient rotational torque is applied, and entirely dependent on the material properties of the two ovals, 'Sticktion' will occur.
This is different to friction in that friction is dependent on the real world co-coefficients of friction of the two materials and the radius at which the friction is applied - surface area has no bearing on friction, as if the surface area is doubled, the specific force is halved.
mdcarson, you may want to consider a purely fluid free solution (by which I mean no greases etc).
If you are looking for a combination of both consistency and longevity in your design then I would recommend that you use a single material as in this way you will achieve a theoretically identical co-efficient of friction between the two components and this will, combined with an identical hardness between the two components, result in a more consistent feel over the course of time as there will be less resultant wearing.
My initial thought would be to use Brass, as it's considered to be a semi self lubricating metal and has good machining properties.
For the level of smoothness you require, keep in mind that machining alone will probably be insufficient to achieve this - the components will probably require surface grinding and possibly lapping or honing as a finishing process.
I would also place a hard rubber washer on the back of each brass component and have two springs (one from either side) compress them together - the reason for two springs is that no compression spring that has closed and ground ends is perfect in its ability to apply pressure uniformly. With two springs, although in theory this could double the potential for uneven forces, it will also halve the force that each spring is required to apply, thus potentially halving the possibility of friction 'hot spots'
Lastly, the tolerance of the shaft that the Brass washers locate on and the tolerance of the I.D. of the two washers has to be very closely controlled - this of course is easier to achieve as the thickness of the washer is increased.
Space, of course, is always an issue....

 

If you use Brass, you'd probably want to look at an oil-impregnated brass. An impregnated plastic washer might do well, as well. iGUS has a line of impregnated plastic bushings that might work for you.

 

Reduced friction

One option that no-one has mentioned so far is that of using one disc in a metallic material that has been micro finished. I am not sure what part of the world you are in but a net search for micro finishing will turn up potential suppliers.

This process greatly reduces the surface finish peaks and valleys and so should reduce the initial static friction as well as the dynamic resistance. The process is seeing use in motorsport and, for example, on camshaft surfaces where a friction free bearing surface is desirable between the cam lobes and followers. These guys are also after the increased fatigue resistance but that is of no interest in your application.

How about PFTE against a hardened and micro finished steel disc?

Lastly, you are trying to replicate the "feel" of the stereo system volume knob or similar so bear in mind that this mechanism uses only a narrow "wiper" that is somewhat spring loaded against the resistance material. Maybe two large area discs with grease etc is the wrong approach and you would be better off running tests that replicate the potentiometer a little more closely??

 

If you use a oil impregnated metal, you must keep in mind that this will have been sintered using a powder metallurgy process - with sintering the individual powder particles are melded together under pressure and temperature to create sufficient cavities for the oil to reside in - but you will still require a extremely accurate finishing process, or you'll be back to square one. Also keep in mind that you are designing a component for use within a highly disparate temperature environment - just as engines run smoother when hot, the same applies to any oil dependent mechanism to varying degrees. I think you will find it hard to achieve consistency using any form of lubricant.

Micro finishing is essentially machine controlled lapping, and if you're looking at a simple linear surface such as the face of a washer, it's very easily achievable, although if production is required you can also consider burnishing using a CNC lathe. Following the final high tolerance cut, use a needle bearing to burnish the component - you should be able to achieve a consistent surface roughness to within 5 Microns.

As I said before, I'd advise against significant material properties disparity between the two contact areas of what is essentially a slipping clutch.

 

K.I.S.S.es original post clarified my issue and it is working great now, almost too well. I used a small HDPE shim washer with a small OD as the spacer and the source of friction. The bulk of the 'contact' area is filled with grease. Im still compressing together with a spring but the gap is now limited. It sat for a week over xmas and still oily and no stickiness at all, in fact I need to add some friction back in so slight amounts of torque dont cause the joint to slowly creep.

 

Hi mdcarson,
Thanks, I think?? I'm not entirely sure how my advice assisted given the solution you've opted for, but in any event, thanks for the courtesy of the reply, as I'm sure all people who attempt to assist forum members appreciate.
And a happy New Year and all the best for 2015 to everyone.