Stick Slip Friction Phenomenon

Maybe replace the grease with a dry lubricant such as:

PTFE Release Agent / Dry Lubricant 3 - Miller Stephenson

Depending on many unknown factors, this may require reapplication, maybe not.

I read you want to "limit" slip. Then why are you using grease which will make the surfaces more slippery? Am I understanding correctly?

I read that he wants to limit stick. That's why the referrence grease and to lower Pv.

My thinking is that the teflon will offer less resistance to the initial movement of the two different surfaces.

Knowing the relative flatness and area and surface finish and pressure per some unit would be good.

Read limit stick. Stick-slip provides acoustic vibration

You need to tell us more.

Please explain the system more clearly. Is the steel moving over a flatbed, slats, rollers(!) - or what?

Don't see how anyone can comment about lubrication, control system or anything else without knowing what the setup is.

Thanks all for the replies, will get more data tomorow. Rollers and balls are considered but do not come at the lowest cost. I will have geomerty, aspect ratios and phydical data to feed your helpfull minds! For starters, this is a stainless ribbon moving in a groove, load is external but close to the groove so there is a cantilever and load concentration, top inside groove, bottom outside groove. Sorry for the no formatting, iphone...

• Maybe you could install some rollers?
• Maybe some small removable tabs to hold the stainless/plastic apart spaced/distributed (reduce surface area in contact) so they can take the load, and be removed later in the process?
• Could you use pressurized air to create a non-contact air cushion between the two?
• I have found UHMW to work quite well as a slide/guide material
• Maybe use a thin film of oil instead of grease?
• Believe it or not... I have found that ordinary Pam cooking spray works well as a mould release agent as well as a general lubricant for many different sliding applications.
• If I don't quit now, I'll ramble off so long no one will want to bother themselves with reading this (not sure why I decided to put it into bullet format).

Could you repeat the last half of that?

I got tired of looking.

I love UHMW, doesn't machine well and can't be injection molded. Air or hydrostatic bearing would be nice, tolerances and fluid power are not available. All ideas are good to have, the ones we missed are bad... TFE loaded oil or grease is in my plans to test.

Something to try then, quite on topic...

Can you vary the speed at all? I was running some tests the other week on graphite and aluminum and found the slip-stick was incredibly sensitive to speed.

Ernest Rabinowicz, the old friction guru from MIT, says you get resonance usually when you have a velocity where the f vs v curve has a negative slope and this usually occurs at higher velocities owing to thermal softening. Since you're using grease at low speeds, I'd try for a different grease, suspecting the grease is starting to pull away from the interface at your load and speed.

Very good points, tks TVP45, I will add these to my list. The grease is probably not staying put, this is a case for heavy loaded grease, TFE or other. Surfaces are so smooth, this is also not helping. Perhaps wicks and light oil to form a packing to run on less polished steel?

I juat discovered an anti stick-slip Nylatron grade which is supposed to have an essentially constant friction coefficients, static and dynamic. A shure try! Not a shure success, the metal part has spring characteristics under the applied load and friction can't be constant along the engangement in plastic. Discontinuous plastic part has been tested but I believe in rather long segments (centimeters), short ones may be better. I get the full data package this morning...

Looks like a pretty good model for fingernails on a chalkboard....

Sounds more like a deflating baloon, a bit better but still a bad trademark...

Yes, really.

Maybe difficult to determine the mode of deformation of the fingernails.

Also:

tires on streets if brakes are blocked,

styrofoam on windows,

break-disks against brake-pads,

railway- or tram-wheels against tracks in curves,

tables on the floor,

and not to forget too flexible cutting tools on workpieces or rigid cutting tools on flexible workpieces or misoriented cutting tools.

(To be continued.)

RHABE

I got a recording, ma be a good prank sound effect...

Rulon is sometimes used in place of nylon for these applications. However, if your system operates perfectly and your only problem is noise from the stick-slip phenomenon, why not consider the use of acoustic noise cancellation technology?

Active technology is out of the question due to cost, but would work fine, the fundamental frequency is pretty clean. The resonant frequency and damping shall be dealt with mechanically.

We need more information, BUT as an initial general comment, look at making the system more rigid. Resonent oscillations will only occur in relation to the 'flexibility' in the bearing system, or in the moving force application system (assuming the structure is rigid!!).

Bearings of the "Glacier" type (eg Lead-PTFE mix in a Steel sheet backed, porous Bronze face) are very rigid (as comment only!).

Try to find a system without the use of grease, it could well be more consistent.

Would brigning the excitation frequency to a higher range be somewhat equivalent to increased rigidity? As stated elsewhere the steel part has spring behaviour which will be difficult to change. The steel engangement in plastic is continuous for 50cm, 5mm segments with 1mm gaps should produce a different excitation pattern?

Maybe your segmented contact approach would be a promising path. The idea is that a lot of little slips instead of a few big slips would be aperiodic and therefore less perceptible. Dissipate the slip.

This reminds me of plate tectonics and earthquakes.

Exactly, the tectonic frequency range would be less of a ear sore...

The origin of the sound may be stick-slip. If this is correct, the frequency of the sound should be a function of ribbon tension. On the other hand you may want to investigate whether machine structure is amplifying the sound as in a guitar or piano.

Yes, the stick slip is the exciter, in this case the plastic part is the rigid member entering resonance, the steel ribbon acting as the loudspeaker. I will try some ways of damping the plastic as well as reducing the stick slip.

As a general statement, yes.

If your example is producing a resonance, and assuming slots are at 90 deg to motion, then try mixing different segment lengths, or mixing slot depths. or putting slots at various (mixed) angles.

There may be a misunderstanding of the info-speak regarding a desire to limit stick slip. Limiting any phenornenon as a goal usually means intentionally producing it under controlled conditions. You seem to be describing behavior you want to eliminate. Is this true? Or it can be used to mean you do not want it, but are resigned to accepting it and want it to be as small as possible. Is this true? Or is there some advantage to having some small amount of stick slip that I missed somewhere?

Eliminate would be best, the 1KHz to 2.5KHz whine it produces is a show stopper.

What about filling the channel with grease, to damp the sound and to replenish the lubrication?

You have grease between the roller and the ribbon already, but apparently not enough or it gets squeegied off so by the time the ribbon gets to the end of the channel it has no grease between it and the roller. Transverse grooves in the roller, as you have suggested already, would allow the grease bath to get under the ribbon.

An alternative to grease might be graphite.

Yes grease gets wiped off, graphite is a good solution to try. What you call the roller is the plastic part? There are no rolling parts, just steel in a groove. The steel gets regreased at each cycle but I bet it lasts less than 25% of the way.

Dear Sir: I have been in the plastic industry for over 24 years and was recently doing some research on the Nylatron 703xl. This product is made by a company called Quadrant. They have locations all over the world and 2 in the US. This material is imported from one of their plants and brought into the US. It is a very good material, and does have zero slip/stick. With this product you will not have to lubricate or grease to keep running. The material was designed so there would be no jerking motions in certain applications. It is being used by many fire truck manufactuers for their boom ladders. As they extended the ladder at a certain height the ladder would start jerking. By installing this the ladder moves smoothly at all levels. There are countless applications for it as well. There are some drawbacks to their material as compared to another material that, in my mind is much better and about 15-20% cheaper. Quadrants material only comes in plate. There is another material, same color, Purple, that is manufactured in plate, rod, and tube. The great thing about this other company, which I have been dealing with for years is that they can cast your part to the exact thickness you need as well as size. For example, if you needed a plate 2.125 inches thick, you would have,to purchase a plate from Quadrant that is 2.5 inches thick and fly cut it down. They also sell the material in full sheets only. I am also a distributor for Quadrant and can buy their material as well. Currently I am working on an application to switch a customer from 703xl to the material I described. I have compared both data sheets and the material I have referenced shows better specs than the 703XL. If you can get a part cast to the exact size and thickness you need, this will save money if the part has to be machined to print. The 703XL is a good mayerial, but there is another choice. The company I work for has branches all over the US as well as Canada. We have been in business for a very long time and I only work major accounts that require a salesperson with excellent technical knowledge. Where would this material need to be shipped to? How many pieces and what size would you need? I also ran a machine shop for a year so I am very familiar with print reading and CNC equipment. If you would like additional info you can e-mail me*. I just wanted to point out that there is another choice, and this issue can be solved very easily. The material is on the expensive side, but the material I have referenced will have a cost savings and can also be cast to your size. Both materials are manufactured by casting, not extrusion.

*CR4 Admin - email address removed

From the CR4 Rules: Do not post phone numbers or email addresses. The CR4 Admin will delete all phone numbers and email addresses posted in threads or comments. You can share this information via the CR4 internal messaging system.

Learn to format your posts. Very few (if any) of us are going to plough through your offering as it stands. I'd sugggest no more than 3 or 4 lines per paragraph.

Also, don't post your e-mail address in open forum, unless you want to attract lots of spam.

As I CBA to read your post as it stands, I don't know whether it's an advertisment for your company. If it is, it will be reported and removed.

Thank you for a hint of a new material, cost is a very important issue.

In the interest of keeping this an open discussion, your disclosing the nature of it would serve the purpose of the community.

I've heard Nylatron XL can be extruded, although I have no real confirmation.

JohnDG's comments have truth, you can message users directly with sensitive information.

FYI, slip is not your problem. Stick is the source of your vibration. Industries that can't afford vibration or stick, sometimes use slip plates which consists of an oil film pumped between the stationary plate and the moving load. A slip plate may be a good model to help in your study.

Here is my long due update, thanks to all that helped. This problem was resolved very quickly and I am waiting for my client to test all identified solutions.

Given the PV and system parameters, I would qualify the observed phenomenon as unconventional stick slip. The Nylatron manufacturer expressed surprise and doubts on stick-slip in these conditions.

The parts put in friction are a plastic cylinder with a helical groove on its periphery and a helical flat steel spring. Load is applied to the spring, the plastic cylinder rotates. as the mechanism operates, it builds a hollow cylindrical column of varying lenght. The plastic cylinder is quite more rigid than the steel part.

FFT noise analysis showed several frequency peaks with frequencies changing several 100's Hz with correlation with the column length (250hZ to 1KHz). One such peak did not change more than 50Hz, 120Hz to 170Hz was considered the excitation frequency.

Sections of the steel helical spring were tested for resonant frequency, a C shaped section placed over the edge of a table and boiing... The excitation frequency range corresponded to a spring length of about 1/3 and less than 1/2 turn.

I imagine the mechanical excitation origin to be due to local discontinuities in the effective contact surfaces and related resulting friction forces. Also, the plastic cylinder and helical spring concentric guiding is loose in the 0.5mm range. So 3D forces loading and release is the vibration system I consider.

Greases with additives renowned to mitigate stick-slip were identified and will be tested.

The first grease, provided as a free sample by Magnalube, is Magnalube-GX. Additives are molybdenum disulfide, PTFE, Lithium complex and expect it to contain graphite.

The application of this grease improved the quiet operation duration by over 1 order of magnitude and brought the product to be within acceptable performance.

Several other grease formulations and grades will also be tested, all containing at least PTFE.

All is good for my client; the solution path came perhaps quicker than expected.