How to achieve an oily-smooth sliding surface that does not sieze up later?
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. [URL="http://maynenkhifusheng.vn/"]May nen khi[/URL] 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