Retainer Ring Shaft Tolerance

When designing retainer ring grooves on shaft ends, do you add a little extra tolerance on the shaft length to make it easier to get on-off and provide a little freeplay? For a 1/2" retainer ring (0.035" thick), McMaster recommends a 0.039" groove width. Is this 0.004" enough?

I typically do just wide enough for it to reliably seat, and not any wider than that. To get them on easily, we typically chamfer the end of the shaft.

+1 point for that simple step.

I’ve never had trouble with the tolerances being off. I’ve certainly cut the rings too deep, and had rattley snap rings, but I’ve never had any trouble assembling them because I was under the correct dimension in terms of width. On the other hand, we don’t cut many snap ring groves.

We’ve just got a cutoff tool that was specially ground down to be the exact dimension of a snap ring grove. It’s saved us a ton of work.

We have an addiction to snap rings.

We have a grooving tool for each specified snap ring width, and haven’t ever had problems with that.

If the ring matches one in a table in Machinery’s Handbook—which it does—I’d just go with whatever tolerance they list. In this case the standard is MS 16624; I’d be completely unsurprised if McMaster was quoting its dimensions from that specification.

(All dimensions in inches; emphasis added.)

So, if you stack up the tolerances of the ring and the groove, it looks like they recommend quite a tight fit in terms of end play.

Realistically, it shouldn’t matter too much, as long as you’re not loading the ring anywhere near its limit. Your case for sacrificing a bit of load capacity for ease of assembly is probably a good one, in that case. If this was holding a wheel on, I would follow the specification carefully.

I suppose, on a smaller note, what’s the rule/recommendation for the amount of material that ought to be left at the end of the shaft? I’m reading ~.05", but not sure what other FIRSTers use.

  • Sunny G.

I’m reading the question differently than most.

It sounds like the OP is asking “Should I add some small amount of clearance to my width between snap rings so when I assemble everything the parts aren’t stacked line to line between the rings?”

The answer to that is yes. Your tolerance stackup will inevitably lead to it being impossible for you to get the 2nd snap ring on without some slop.

You do not want that slop to be in the groove though. You should cut the groove .039 like McMaster says. Just add in .005-.010 to the distance between grooves.

Yes this is what I was looking for… thank you! I should have been more clear… my example is that we are designing a new transmission and the tolerance stack up of all the gears, sprockets, bearings, frame, wheel, etc along the main output shaft may add up to the point that the end ring wouldn’t fit. There is a spacer we could shave down to most likely solve any issues but wanted to avoid having to machine a longer output shaft. We then started to wonder about the tolerance stack up for a simple WCD axle (sprocket, bearings, wheel, etc). I couldn’t find any manufacturing tolerances for most of the parts to make an estimation of what a worst case would be. 0.005-0.010" sounds good.

Here is a tip when assembling our transmissions. We use fiber washers as shims to get the spacing just right between parts on a shaft.

We were thinking of using a laminated washer like:

http://www.mcmaster.com/#round-laminated-shims/=k1q52f

Does the fiber hold up well? Maybe adds a little give?

My absolute favorite for handling the slop take-up is a wave washer. Industrial robots use them everywhere in their gearboxes and the premier industrial gearbox companies that sell to industrial robot companies all use them. The McMaster versions are located here: http://www.mcmaster.com/#wave-disc-spring-washers/=k375io

These work especially well for gearboxes using spur gears.

Paul