Tapered locking pin, and ejection due to radial force 1

[adventg]

Morning all,

Long time lurker of these forums, first time poster today however.

Currently got a little bit of an issue, I am currently reworking an existing design in which a parallel pin drops into a parallel hole within a sliding component, used to lock it in place.

I wish to remove play from the system as far as reasonably practical, and having experimented with highly toleranced parallel parts - they are simply far too sensitive to contamination, tolerance stack-up etc in this particular application, and thus I'm looking at making the portion of the pin which locks the sliding component into place, tapered - and thus the hole into which it will seat, tapered also.

I've got some concerns about this, in that these pins are subject to a large radial force - and it seems likely to me that a tapered pin, in a tapered hole subject to large radial force will try to 'self eject', i.e. back out of the hole. I don't really have time to physically test, before I need to physically retest the entire assembly.

What is ones input on this, the only reasonable logic I can seem to apply is that if the total taper is less than the critical locking taper angle, perhaps these won't eject. Large angles obviously would eject... I only have room infact for a very shallow angle anyway. They are spring loaded, but only lightly (~20N).

Just looking for a little bit of opinion on this?

Many thanks in advance!

 

[desertfox]

Hi

Please post some sketches!

 

[adventg]

Okay, I've attacheded an image of not the actual geometry (cannot release) but of a representative scenario.

So, the blue block is fixed, the grey pin slides up and down in the bore in the blue block, and is spring loaded downwards on the image. The green block slides relative left and right to the blue block, and previously was locked in place using a parallel pin arrangement.

If the pin were to be tapered, as per attached image - the force applied to the green block which imparts a radial load on the pin - would this cause the pin to be ejected, this is the concern.

The pin is not hammered into place as with a tapered dowel pin, it is merely a lock pin, which is released by means of a pull cable from above.

 

[Tmoose]

If the left-right force varies much at all I expect the pin will be worried loose after a little while in service. The potential for pin and green block wear is significant. The fixed blue block hole may not fair well either.

If the blue block is fixed I'd replace the cable with something that could apply a significant "push" force as well as "pull."

It looks to me The pin is shown bottoming in the hole. In order for the taper to positively engage, the tapered hole will need to be much deeper.

How quickly must this cycle?

 

[adventg]

The left-right force is only a very rare occurence in large magnitude, the cyclic stresses are very very low.

Block wear is accounted for, as the pin and the bores are in fact hardened steel sleeves (the attached image is massively simplified). The blue block is fixed, but for many a reason - only a pull-cable and spring can be used here.

The pin in the real application dosen't bottom, as it's a through tapered hole. Cycle wise, once every couple of days or so.

I'm just running some FE analyses, with moving contact sets and friction elements enabled - which suggest at low angles the pin will not eject, but I'm still not entirely convinced.

 

[BrianE22]

If there is enough vibration in the vertical direction then the inertial acceleration of the pin could cause loss of contact and the "effective" coefficient of friction would be reduced to zero.

 

[adventg]

Brian, thankfully this problem is accounted for - the spring pre-load is enough to keep the spring seated well above expected service accelerations.

 

[racookpe1978]

Do you have to disassemble, or can you tack weld the pin?

(The weld weight will affect balance.)

 

[adventg]

No, as per original post - the pin is a pin that needs to be retractable to allow for positional adjustment of the lower block.

 

[GregLocock]

The correct part for use in that application is a cotter pin, a tapered pin with a thread on the small end. Since I imagine that is not desirable, thread the blue hole and stick a bolt down it to keep the taper pin in place under preload.

Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376

http://eng-tips.com/market.cfm?

 

[racookpe1978]

yes, good idea. Tap the blue hole, but use a setscrew (not a bolt though) to lock the blue screw in place against the green sliding pin.

The set screw will not extend out past the rod OD, and will be a predictable lower weight for the balancing problem.

 

[desertfox]

Hi itw11

To answer your concern directly the angle of the taper will be the main factor on whether the pin moves upward under radial load.
When the radial force is applied to the pin it generates vertical and horizontal
force components, now the pin can only move upwards if the vertical component of force from the radial load is large enough to overcome the friction generated between the pin and mating hole and the spring force your using to hold the pin in place.

The concern I would have about the tapered pin is one of alignment between the two components,IE how good is the positioning of the two components for the pin to engage properly?
Is the spring force large enough to drive the pin into the hole and physically move the two components into proper alignment should they be lightly misaligned prior to pin engagement.

https://books.google.co.uk/books?id...age&q=wedges in engineering mechanics&f=false

Have a look at the link above dealing in the mechanics of wedges

 

[3DDave]

There is a thing called a self-locking taper. Tapers below the atan of the friction coefficient a taper won't tend to back out.

Some new-grads I worked with made a bracket with maybe a 5 degree or less taper and included a pull-down screw to really make it secure. Then needed a hammer to drive the mating part out of the bracket.

Here's a conversational discussion. It may be a good place to start

http://www.practicalmachinist.com/vb/general-archive/ot-theory-self-holding-taper-79627/

 

[adventg]

Cotters pins, bolts and welds etc are all not suitable in this application, given it needs to be cycled daily, and in a non accesible location.

racookpe1978, I think you may have missed something in the problem - balance is not an issue at all in this application.

Simulations seem to suggest even fairly steep angles will not eject the pin here, as I also have the friction of the parallel pin in its bore resisting ejection.

Will feedback on this.

 

[1gibson]

Do you have room in the outer part to make a small o-ring groove, and rely on the o-ring compression to hold the pin? Still easy enough to get in/out, but you'll have some resistance to moving by itself. Of course you'd also need room for a flat bore and flat pin section near the OD.