taper pins or roll pins

Has anyone ever used roll pins instead of taper pins for a piston rod/ crosshead fixing. I already have some roll pins but not any taper pins or reamers.

No. But I can't see any reason that it wouild not work, as long as the pin is not load bearing and is just retaining the load bearing pin itself.

I think it would be fine and here is why. I was finishing a part built loco for a young bloke. Rushing to get to the exciting testing on air, I merely pinched the piston rod with an M3 grub screw through the side of the crosshead. The rod was a good close sliding fit done by the original builder, whose workmanship was a pleasure to take over (rarely the case!). This was forgotten as the engine was finished. It has been running for years, pulling heavy loads. Still just the little grub screw. So a roll pin would be luxury! There is theory and there is practice.

I suppose they will do in any situation, as Hopper suggests.

My view: taper pins are the engineer’s solution to a fixing challenge - one strike to secure and one to free them, for a basically precision job, whereas the roll pin is the fitter’s option - need to be hammered all the way in and hammered all the way out and can be fitted to a (roughly) drilled hole!🙂

Verify that you have decent quality roll pins. I'm underwhelmed by the ability of single split ones to stay properly round and secure.

Especially the modern "affordable" box sets. My almost affordable box set was got many years ago and said to be of engineering quality but I've had trouble with pins crushing slightly out of round and loosening under quite moderate loads. Some of the "can you fix it" jobs have roll pins in truly horrible condition. Apparently factory fit too.

It appears that it should be possible to re-use them after suitable inspection but I have my doubts.

Spirol types seem much better performers and probably worth the extra cost. Its what I shall buy if I ever run out.

Clive

Problem is, a roll pin is (or should be) hardened and this, combined with its asymmetric shape, makes it almost impossible to drill out if it fails to drive out properly when dismantling is required. DAMHIK.

Subjectively, taper pins seem more difficult to remove (particularly if struck from the big end!).

A possible advantage of the roll pin may be that it is easier to arrange it not to project at either end of its hole.

I have no doubt roll pins are heat-treated in some way but are they really hardened?

Personally I wouldn't use either as they're not really prototypical. The ideal would be a tapered gib as per full size.

Andrew

Thanks for the replies guys. I will go down the roll pin route especially after reading about Simon's M3 grub screw. Going down the ENGINEER/FITTER road :- despite machining as accurately as possible I am finding that I am having to use lots of marking blue together with careful use of the needle files and a few thin shims to get things moving reasonably smoothly. This is my first loco (Pansy) so am not sure if this is normal or not.

Hello Peter,

Sorry for the delay in replying.

I personally would never use roll pins on the motion, and especially the crosshead to piston rod. Why? Because I've had to remove them for an overhaul, and with great difficulty - with the battering ram effect of piston rod betwixt crosshead distorting them.

A solid fixing is preferable, via a taper pin IMHO.

Cheers,

Julian