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  • Press Fits in Nylon

    I have a pallet truck, that's had a hard life. The tyres on the front rollers have finally given out, so I'm in the process of repairing them. I've cleaned the old nylon off the rollers and skimmed them clean; they are now steel, 63mm diameter, 100mm long.

    I have some white Nylon bar that I'd like to use to make the new tyres; finished outside diameter will be around 83mm, giving about 10mm thickness Nylon on top of the steel rollers. This is how the original tyres looked.

    I'll machine a bit of a profile (ie. surface roughness) in the bores of the Nylon tyres, and then coat both surfaces with epoxy prior to assembly. I don't think the epoxy will stick to the Nylon too well, but if it sticks to the steel, the profile in the Nylon should be enough to help with holding them in place. The old tyres appeared to be bonded to the steel.

    The end result has to be pretty solid, or the tyres will just walk sideways off the rollers.

    My question;
    I'd like the steel rollers to be a tight press fit in the Nylon tyres. How much undersize should the bores in the Nylon tyres be? I'm seeing this as more of a stretch fit than anything - but I have no idea what a reasonable amount of stretch would be for the Nylon.

    Anyone know?

    Thanks,

    Ian
    All of the gear, no idea...

  • #2
    Don't make it TOO tight or you'll split the nylon.

    I've done a bit of this sort of thing and on a 63mm diameter I'd likely shoot for something like .07mm to maybe as much as .1mm. Anymore than that and I'd worry about the nylon splitting.

    For this sort of use though I have to wonder at what the load will do. You're pinching the nylon strongly between the steel hub and the floor. Over time I can see the wheel stretching and coming off due to this load and losing the fit. Usually we're using something like nylon as a bushing or a low pressure wheel. But hey if it's what you have then you may as well go for it.
    Chilliwack BC, Canada

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    • #3
      Hi BC,

      Thanks for that. I had a bit more stretch in mind - about 1mm undersize, but as you say, splitting the Nylon's the risk. The originals were also made of Nylon - this was obvious when I machined what was left of it off. I know exactly what you mean with the stretching when in use, but the originals lasted about 20 years under all sorts of abuse. Load-wise, the truck is rated at 2 tons, so something around 500Kg per wheel if evenly loaded. In reality, the small front rollers take most of the load, probably 750Kg each.

      Ian
      All of the gear, no idea...

      Comment


      • #4
        To stretch that far the hubs must have a taper on one end or they would shear off material. Or they used a tapered mandrel to open the tires up as they were pushed on. Is there a taper on one end you can use as a suggestion of the amount of stretch?

        I like your idea of cutting a few small grooves in the nylon for epoxy. As you say it won't stick to the nylon worth a hoot. But it would bond to the metal and form ridges that stop the nylon from walking sideways. And that should greatly reduce the need for a high amount of stretch.
        Last edited by BCRider; 01-07-2018, 01:27 PM.
        Chilliwack BC, Canada

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        • #5
          Nylon's mechanical properties vary wildly depending on what grade you have and how dry it is.
          Unless its known crap 5% elongation before yield and 20% elongation before break looks typical.
          2% stretch on press fit would look reasonable if 5% is the yield limit..so your 1mm interference fit on 63mm steel shaft is still conservative.

          Dupont gives similar ideas, graph 9.10 suggests 2 to 4% maximum interference fit.
          http://www.dupont.com/content/dam/du...20Snap-fit.pdf
          Location: Helsinki, Finland, Europe

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          • #6
            Above pdf also gives other good guidelines that you might apply:

            "As previously stated, a press-fit joint will creep and/or
            stress relax with time. This will reduce the joint pressure
            and holding power of the assembly. To counteract this,
            the designer should knurl or groove the parts. The plastic
            will then tend to flow into the groves and retain the holding
            power of the joint."

            I'd consider grooving or cutting a partial thread profile to the steel part if possible.
            Location: Helsinki, Finland, Europe

            Comment


            • #7
              One of our pallet jacks at work is a low profile unit. As such it has smaller diameter and uncoated steel front rollers. It is noticeably easier to roll around with weight on it than the standard pallet jacks. I would leave it as is. If the diameter is now too small, make larger diameter steel rollers.

              ME

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              • #8
                Have you priced new rollers? I used to buy them for a product I built and no way I could make a wheel at that low a price. McMaster Carr sells a wide variety to fit all kinds of trucks.

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                • #9
                  Watch out for a humid day. The nylon tires will swell and fall off.

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                  • #10
                    Polyurethane is better. I made some years ago.. still working fine.

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                    • #11
                      Hi all,

                      Yes, some good stuff there.

                      I'm tempted to go the solid metal route, I must admit; my workshop floor is dead smooth concrete, so would be fairly ideal for that - cheap steel bushings would do the job. I'll start with the Nylon, simply because I have 2 lumps all ready to use. If this fails, I'll go with metal.

                      Knurling sounds like a good idea - I was going to rough the steel up with a really course grade of emery (40 grit), but a knurl would probably be better. I've already chamfered one end, ready for a press fit.

                      Lakeside, polyurethane; HDPE? I had the idea that Nylon was harder, but there are so many grades of both.

                      Gary; yes, I looked at new ones. I'm having difficulty finding the right size (here in Holland). The spindle is an oddball size - just over 16mm. Most of the rollers that I found had 20 or 25mm shafts.

                      Matt, thanks for the Dupont document. Going by that, I need to bore the Nylon tyres to 1.9mm undersize, which kinda feels right. I'll go with that I think, along with epoxy and knurling the steel, roughing the Nylon surface up with 40 grit. Let's see if it holds...

                      Thanks all,

                      Ian
                      All of the gear, no idea...

                      Comment


                      • #12
                        Originally posted by Ian B View Post
                        Gary; yes, I looked at new ones. I'm having difficulty finding the right size (here in Holland). The spindle is an oddball size - just over 16mm. Most of the rollers that I found had 20 or 25mm shafts.
                        Really? That's the big roadblock? Wouldn't a steel sleeve fix that? Should not even need a press fit, a smooth sliding fit should be adequate. Can't
                        make a 100mm sleeve? Make four 25mm sleeves and slide them in.
                        Location: Long Island, N.Y.

                        Comment


                        • #13
                          Hi Rich,

                          Yes, I'm sure I could knock a couple of sleeves up. It's just that the HSM disease kicked in; why buy something when for twice the price, you can make something half as good

                          Ian
                          All of the gear, no idea...

                          Comment


                          • #14
                            Well, as long as it's all about the journey and not the destination, then OK.
                            Location: Long Island, N.Y.

                            Comment


                            • #15
                              Originally posted by Ian B View Post
                              I have a pallet truck, that's had a hard life. The tyres on the front rollers have finally given out, so I'm in the process of repairing them. I've cleaned the old nylon off the rollers and skimmed them clean; they are now steel, 63mm diameter, 100mm long.

                              I have some white Nylon bar that I'd like to use to make the new tyres; finished outside diameter will be around 83mm, giving about 10mm thickness Nylon on top of the steel rollers. This is how the original tyres looked.

                              I'll machine a bit of a profile (ie. surface roughness) in the bores of the Nylon tyres, and then coat both surfaces with epoxy prior to assembly. I don't think the epoxy will stick to the Nylon too well, but if it sticks to the steel, the profile in the Nylon should be enough to help with holding them in place. The old tyres appeared to be bonded to the steel.

                              The end result has to be pretty solid, or the tyres will just walk sideways off the rollers.

                              My question;
                              I'd like the steel rollers to be a tight press fit in the Nylon tyres. How much undersize should the bores in the Nylon tyres be? I'm seeing this as more of a stretch fit than anything - but I have no idea what a reasonable amount of stretch would be for the Nylon.

                              Anyone know?

                              Thanks,

                              Ian
                              I haven't read all the replies so ignore me if I'm off here. What you could do to prevent the tires from walking off the steel wheels would be to machine a small ..say 1/8" wide x 1/16" deep groove in the center of the ID of the nylon tire and the same size groove in the steel wheel. Then drill two small holes in the center of the nylon wheels groove, 180 deg. apart and pump some Loc-Tite type material into the groove, thus internally locking the two parts together. That nylon tire would never slide off the steel wheel.
                              For those of you that may remember when GM started doing this with their U-joints to save the expense of using circlips to hold the U-joint in the yoke. They were a PIA to remove.
                              You might even want to machine a couple grooves in each wheel for reassurance. This method of locking the nylon to the steel wheel would eliminate the guessing of how much of a press fit to go with. I'm not sure what type of injectable loc tite type products there are out there, maybe someone else would know what would work best.

                              I'm not sure how the originals were secured to the steel wheel. I've never heard of vulcanizing nylon to steel.

                              JL................

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