|Clive Foster||11/09/2019 08:04:20|
|1992 forum posts|
As the load flow is circumferential round the outside of the shaft and inside of the bore any drilling or pinning will cause stress concentration where the load lines scrunch up to go round the pin and its hole. Which can make afair difference to maximum load capacity.
As ever the more you look at it the more complicated it gets if accurate answeres are needed. Probably best approach is to follow standard, known to work, practice and run the numbers to ensure you have a decent factor of ignorance.
If you fancy a taper press fit, motorcycle stye the Velocette self gauging system is relatively easy. Basically the two tapers slide together half way before pressing on. Still got to do the maths as to load carrying ability, roughly proportional to the engged area of the taper, and hoop stresses around the female taper to be sure that it won't stretch. Riveted and pressed BSA system is a good example of the pros not getting it right. The DB34 Gold Star is known for wallowing its drive side shaft loose. Especially when used on the road witha doube R box. The basic parallel press fit of the shaft in the flywheel is actually slightly heavier than the Velo taper system giving a theoretically stronger joint. But Velo drive side shafts stay put unless seriosuly abused but Goldie ones will loosen. For all practical purposes of power performance and general engine loads the Thruxton is the "same" as a DB34.
Edited By Clive Foster on 11/09/2019 08:05:02
|Martin Kyte||11/09/2019 09:14:29|
1570 forum posts
So far I think the case for the use of pins in a loctite joint is not proven , at least to me. Couple of comments.
Yes keyways would be a lot stronger but we are talking about diametrical pins here.
Drilling for a pin reduces the area of adheasive contact and so reduces the strength of the bonded join in my example by about 5.5%.
You cannot possibly have equal load sharing unless either pre-stressed or the bonded joint can move, so were are still in the realm of the bond failing followed by the pin.
Personally I feel that the thinking may be somewhat historical. If in the past you were going to pin a crank and rely just on the pin 'gluing it all together with loctite holds the thing together whilst the pinning operation takes place. In this scenario we would be relying on the strength of the pin and any chemical bond is just a bonus. Early chemical bonds were really not as strong as current offerings and it takes time to build confidence in new techniques.( especially for us). Perhaps a s time went by the bonded joints became stronger with better the introduction of better products. and the pinning was done because that was what was always done. I don't know, but I'm interested in comments.
I do think the peace of mind statements are valid, just but for psychological reather than engineering reasons.
Finally how catastrophic is a joint failure on a miniture loco?
|5134 forum posts|
My take on pins and retainers like Loctite is that neither are ideal for this job - a pin takes all the stress on a relatively small bit of metal and is likely to sheer, while the retainer is a chemical bond liable to be weakened by dirt or if moved while setting. Adhesives are also effected by sunshine, excessive heat, and solvents, including oil and water. That said, both could well be 'good-enough' to outlive the builder!
I thought adhesives made weak joints until I followed the instructions! Extreme cleanliness - a quick wipe with a cloth isn't enough, especially if any oil is present. Fussing the joint into position while the adhesive is setting is bad news, as is stressing the joint before the glue has fully set. Leave it alone!
The advantage of pinning is it's quick, easy, locates the two parts positionally and can be dismantled. But pins aren't strong. Done properly adhesives are a good way of making a strong joint with minimum fuss, but the parts aren't positively located during assembly and dismantling is difficult.
When fixing a crank to a shaft, I'd see the pin as being used to fix the angle but have strength provided by the adhesive. If the pin is strong enough on it's own, no point in using glue.
If the joint is to be dismantled, and a pin isn't strong enough, look to keyways or splines. Much stronger but far more trouble to make.
Thinking about the failure modes of a combined pin and retainer joint has left me deeply confused. I can't even decide if they share the load like strands in a rope, or if one does all the work such that its failure then stresses the other. It might depend on which strains first under load. The glue and the pin will both move when force is applied and probably at different rates : if the glue gives more than the pin, then the pin will share the load, otherwise the glue will protect the pin. Perhaps!
But back to failure-modes. In practice, I think sub-optimal glue is more likely to fail due to indifferent preparation and its environment than the pin. If a glued joint is slightly dirty, takes too long to position during assembly, and is then overheated, the joint is at risk.
Cranks in reciprocating engines are subjected to dynamic shock loads. The maths is beyond me, but I've read any calculation of safe values based on static loadings should be halved at least.
|Nigel Bennett||11/09/2019 11:31:01|
313 forum posts
I had the wheels come off (well not absolutely off, you understand) my 5"G Edward Thomas a few years ago. The Loctite gave way as a result of some over-large stresses caused (I think) by a hydraulic lock and as a result, the motion ended up in an interesting arrangement of angles and unsurprisingly it Wouldn't Go. I removed the coupling rods and ran it as a 2-2-2 for a few laps, rather than waste the nice bright fire, but it sounded a bit off-beat.
Later investigation showed the quartering on the driving wheels was about 120 deg not 90. I fitted some axial pins once I'd got it all back together.
The load on the Loctite had obviously gone far beyond expectations - but I've never relied solely on Loctite after that for sticking wheels on to axles. Crankshafts would, I imagine, be prone to similar unexpected stresses when in use.
I suppose it's down to where you want the weak point to be - did the Loctite failure on my ET wheels save me knocking a cylinder cover off or bending a rod or two?
|Martin Kyte||11/09/2019 11:36:57|
1570 forum posts
So not much harm done then. How was the crankshaft constructed?
|CHARLES lipscombe||13/09/2019 22:56:12|
|104 forum posts|
Your reference to the tapers holding on a Velocette but not on a Gold Star is very interesting but would you be prepared to speculate on why the Gold Star fitting is less satisfactory? Is the Velo set-up longer? Just idle curiosity on my part
Are you by any chance the son of the great Bob Foster of New Imperial TT fame? If so, we have corresponded ages ago re my using some photos for my book on New Imperials
|Howard Lewis||14/09/2019 00:02:46|
|2733 forum posts|
Just to give a little hope; a large number of us drive around in vehicles where the final drive gear is secured to the differential housing with an anaerobic sealant.
Also, some time ago, I secured an ER collet chuck to the backplate with anaerobic, and knocked it about until the internal taper ran true. Some tears later, I thought that removing the bolts and hitting the body would separate it from the backplate. I gave up when I expected to do some permanent damage to the collect chuck.
And that was having used Loctite which was past it's use by date by several years!
|Stephan Gaal||16/09/2019 05:36:44|
|4 forum posts|
I think the biggest mistake made with loctite is not leaving enough room for it. You can't expect it to hold things together if you squeeze it all out during assy. Having said that I still use a pin at the end once I am sure everything is where it is supposed to be. Perhaps I am a cave man but I would rather have a pin and not need it than the other way around. To date none of my wheels have moved on their axles.
Please login to post a reply.
Want the latest issue of Model Engineer or Model Engineers' Workshop? Use our magazine locator links to find your nearest stockist!
You can contact us by phone, mail or email about the magazines including becoming a contributor, submitting reader's letters or making queries about articles. You can also get in touch about this website, advertising or other general issues.
Click THIS LINK for full contact details.
For subscription issues please see THIS LINK.