material required to support a thread

I am considering tapping an 8BA hole for a grub screw to more securely fix a return crank to its crank pin. The steel crank pin has a thickness of 0.114" . 8BA basic major diameter is 0.0866" and basic minor diameter 0.0661". Is there enough material in the thickness of the crank to support the thread?

Why use an 8BA grub? Because I've got some, but none smaller.

Are there specific rules which govern these situations or even a rule of thumb guide. I have bought and read Tubal Cain's "Drills, Taps And Dies" which covers a lot of ground but it does not seem to address this point. I am conscious of usually going over the top with the size of fasteners that I use......sort of try and work out a size and then go for the next biggest just to be on the safe side.

Any advice will be gratefully received.

Mick

For metric threads( which have a similar thread form to BA) the full thread strength is obtained by achieving 1 1/2 D contact I.E. .15" in the case of 8BA.

Thanks for that answer Roy......it looks as if I might be skating on very thin ice at 0.114".

Having just reread my original post, it is the return crank that is 0.114" thick, not the crank pin.

I wonder also how much "support" the grub screw and its thread would give to the crank pin material when loctited in position?

Mick

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Mick,

If, by that, you mean that the crank pin is to be Loctited in place anway; then the answer should be something near Zero ... I wouldn't bother with the grub screw.

MichaelG.

If you are worried about the Loctite joint 'moving' Mick (and I've not had this happen) why not just drill through and pin it? It would make any subsequent removal that much harder to do though - as the Loctite-only joint can be simply broken with heat if required.

Regards,

IanT

Ian and Michael G......thanks for the interest......no, I did not really want to Loctite the return crank on to the crank pin in order to facilitate future adjustment and the same situation arises with putting a pin through the joint although both have occurred to me as quite attractive options which may need to be taken up at the end of the day. When I mentioned Loctite I should have said that I intended to use low strength threadlocker to secure the grub.

The plans call for the return crank to be 3/32" in thickness fitting on a 1/8" crankpin and secured by a 10BA screw squeezing the end of the crank onto the crankpin. (Sorry, don't know the correct term for that sort of joint). I have made the crank a nice snug fit on the crankpin and all seems well on first assembly but a little usage soon allows the joint to move with a bit of finger pressure even though the motion work is quite free moving. The clamping action of the 10BA screw does not seem to be enough for the job. Hence I am intending to remake the cranks slightly thicker at 0.114" and use an 8BA grub through the crank and acting on the crankpin.

Roy.....when you say 1.5D.......is that major, minor or core diameter?

Mick

A return crank will have a cyclical forward and reverse load imposed upon it so I do not think a simple clamping grub screw of any acceptable size will be satisfactory. Better to experiment with Loctite it to get the correct position and then use a taper pin to lock the crank to the shaft. as Ian T says a Loctite can easily be loosened by heat.

Eric

<condensed quote>

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Thanks for the clarification, Mick

The low strength threadlocker should certainly prevent the grubscrew working-loose.

I'm struggling a little to follow the description of the joint ... could you maybe post a sketch?

MichaelG.

Mick, The rule of thumb for thread strength as mentioned in my reply refers to the outside dia of the thread, so for

example for a 3mm thread any more thread than 4.5 mm will not make the fixing any stronger. However, this really

only applies under tensile stress so perhaps it is not too applicable to your situation where shear forces seem to be your main concern. Perhaps you could modify the grub screw to a dog point?

I think this is what Mick means:

This is a bit bgger though. The crankpin is 0.1875" diameter and the return crank is 0.125" thick. The crank is pinned with a 0.0625" dia. silver steel pin put in after the crank was set.

John

Thanks John,that is exactly the sort of joint..., and a very neat job too. What is the size of the clamping screw that you have used? Did you drill for clearance all the way through the return crank or did you tap the crank and nut it as well? How on earth do you drill for the silver steel pin without breaking through into the coupling rod? Or do you only drill part way through the return crank?

Roy....thanks for further explanation of the 1.5D rule.

Mick

Thanks John,

All's clear now

... Thinking about a possible solution.

MichaelG.

Hi Mick,

I think it's 10BA (the loco's tucked away in her box). One side of the slot is clearance and the other side tapped and the nut is just a lock nut. The bolt is just an ordinary steel one so you can't tighten it too much without shearing it. One made from a stronger material would be better. The return crank is a tight fit on the end of the crankpin to begin with.

The pin goes half in the crank and half in the crankpin and is 0.125" long (the thickness of the return crank. Actually, the end of the crankpin is reduced from 0.25" dia. to 0.1875" to take the return crank so there is no danger of drilling into the connecting rod if you are careful. The crank is made from gauge plate (I use gaugeplate for all valve gear parts as it's hard wearing) and is a similar hardness to the silver steel crankpin so the drill doesn't wander when you drill for the locking pin. The drill can wander if one material is softer than the other.

One of the cranks did come loose last year and the pin fell out when I was pulling myself and another person but it had run for 5 years with no problems until then.

The loco is a 2½" gauge Helen Long by the way.

John

Mick,

I think I can explain how the type of clamped joint illustrated by John might fail to live up to expectations:

For the moment, ignore the little dowel-pin and consider just the clamping action.

Assume that:

• The crankpin is perfectly smooth, straight, and cylindrical.
• The hole in the con-rod has been bored to a close sliding fit on the pin.

You then tighten the cross-bolt, to apply clamping force; and it immediately goes very slightly oval ... with the result that there is [at the microscopic level] only two-line contact.

Solution: Relieve those two areas of the bore very slightly [just a few thou'], and the contact lines will be redistributed. If you happen to get it right [which involves more luck than judgement, I fear] then you should get lines of contact at about 12, 4 and 8 o'clock, with the slot at 6. This is a much more secure and stable arrangement.

Note: In addition to it's obvious function; that little dowel-pin that John shows may well serve to "force" a contact line at 12 o'clock.

... I hope that all makes sense.

If it's any consolation; the problem described is, I believe, a fundamental failing of this design of clamp.

MichaelG.

MichaelG .....I reckon that you are dead right on distortion taking place and in fact it can be seen in the area of the return crank where the cross bolt is fitted. I believe that this is mainly due to the fact that there was so little material left after drilling clearance for the bolt, albeit only a 10BA......bearing in mind that the crank is only shown as 0.09375" and the clearance drill is 0.0689" (1.75mm) but I used 1.8mm which makes matters slightly worse. As mentioned above I can just squeeze 0.114" for my Mk2 cranks which should improve matters slightly. As John B says the cross bolt cannot be tightened too much without danger of shearing so I think I will go with gauge plate and a locking pin through crank and crankpin as mentioned by Ian and Eric......I will just have to make sure that I get the timing spot on (I think it is) before I drill through.

Thank you all for your advice which is greatly appreciated and which has provided several strategies which will, I am sure help me out of trouble in the future.

Mick

With reference to line contacts and possible distortion pin in bore mounts used in optical labs have so called double bores with a second, small, bore opposite the locking screw. Typically the small bore is around 1/4 the diameter of the main bore. For the common 1/2 inch (ish) pins the second bore is drilled and centred on the main bore circumferance but the larger sizes eg 1" and 2" (ish) are bored to put the outside around 40 to 60 thou beyond the main bore. The pins are an easy sliding fit but even with only a simple nylon tipped knurled head locking screw they are held firmly in place with only finger tightening. The two line contacts opposit the screw ensure that the pin is vertical with no rock.

Some brands of larger mount use a pair of floating collets inside a transverse bore rather than a split clamp or radial screw. One collet is threaded, the other plain with the ends suitably tapered so that when the screw is tightened up the taped bear against the pin and force the collets out against the inside of the transverse bore. If anything other than modest torque is applied to the clamp screw these things can jam up so tight as to be almost impossible to remove. Not enough room for the OP to use this idea but worth remembering for a suitable occasion.

The simple bolt and split clamp system inherently puts bending stress on the bolt which tends to reduce the security, especially with slender fixings. If you consider the forces involved when the clamp tries to slip its fairly clear that there is an opportunity for the bolt to "run away from the load" by bending slightly. I've disassembled a fair few larger, slackly toleranced, set-ups of this style where the pinch bolt has bent by overtightening in an attempt to get things to hold.

Clive

Clive,

That's an excellent description of the "double bore" and I agree completely regarding its merits. The saddles on my ancient optical bench have exactly that arrangement, and it works beautifully.

For Mick's application, it would be neater to just broach-out a shallow recess; but the geometric principle would be the same. ... Much more satisfactory than the clamp.

MichaelG.

Any chance of a quick sketch of the double bore, gents? I am sure others would be interested as well.

Mick

Mick,

Here is a quick sketch to show the general concept

The key to making this work, is that the two holes are exactly parallel; so the edges act rather like a Vee block.

MichaelG.