Gallery, Projects and General > The Design Shop
Heat shrink fitting and crankshaft balancing.
bob ward:
I'm late to this thread, but FWIW I've been trying to achieve heat shrink fits on 10mm dia items (there are other ways to do what I'm trying to do but I've not played with heat shrinks before so I thought I'd give it a go) and have abandoned the idea.
Achieving the required dimensions for the male and female was not a problem, neither was heating the items to the correct temperature. Where it all came (un)stuck was in fitting the parts together, in 12mm deep holes I could never get the male part in more then 8mm before it jammed, and jammed forever.
What I think was happening was that given the tiny air gap between the 2 parts and the small mass of the male part, radiant heat from the female part was almost instantaneously growing the male part. I'm not saying you couldn't make heat shrinking work on something this small, but you would need a setup where you plant the male into the female like an absolute rocket.
Lew_Merrick_PE:
It appears the original post has been answered with do it a different way. However, the questions about the strength of an interference fit and how to calculate expansion of part when heated was not answered.
Both Machinery's Handbook (and the imitators) and Shigley's Machine Design have sections on calculating the strength of an interference fit joint. You are basically creating a localized yielding of the two parts which creates a preload on the joint. There is a section in Shigley's Mechanical Engineering Design that has exercises in making these calculations.
You can find exercises on thermal expansion in most mechanical design engineering texts. Shigley's Mechanical Engineering Design is one of the best. You end up needing to know the coefficient of thermal expansion (CTE) for your materials. http://www.matweb.com has the most complete and verified listing of such properties. The CTE has units of length/length/degree of temperature (i.e. in/in/°F or m/m/°C or some variation thereof). Be sure your units are correct.
I hope this helps.
madman:
Crank Pinning... or tig weld after Alignment Completed. If you drill small hole bisecting the crank and crank pin and slight press a small pin it will Not move, Then bonk the hole with a centre punch (120 degrees apart) to keep pin in place. Ive done this on Cranks for my Motorbike engines. As for balancing I do know People make up bobweight assemblies (adjustable for different weight of Lead shot cylinder hollow with threaded end cap and fill with different weight lead shot until ideal operating rpm range and smooth running is achieved) and factor in a percentage of recipricating weight (like a 68 percent factor) then get a guy who knows what hes doing to balance it. I would also like to know how to balance one. I do know that years ago on Honda 750 engines we modded, we hacksawed the crank throws and filed sanded smooth. It never seemed to effect the RPM at high RPMs. We built 1000 cc plus alcohol engines from the Base 750 Hohda SOHC piles we had. I was always suroprised that our Crankshaft Hackery didnt shalke it apart at HIGH RPM where we always ran them.
picclock:
Hi Madmen (welcome to the forum)
That sounds like an easy way to do the job. I'm finding it a bit difficult to visualise the method you wrote about. Sketch below is my interpretation, although inserting the pin from the top into a blind hole may be better (but only if you do not want to remove it :hammer:).
Peening over the edge of the hole sounds like a foolproof way to prevent pin escape, although if I go this route I may try to find some small interior circlips to retain the pin.
It would be nice to be able to calculate the balance mass required as its a lot easier to change the size of the crankshaft webs during manufacture, so getting a close approximation would be useful. The lead shot idea seems good to me for fine tuning.
many thanks
picclock
RichardShute:
There are plenty of (mostly very technical) texts on machine balancing. There are a number of considerations, primary balance and secondary being the fundamentals.
Taking a simplistic view of what I can recall of a subject I've not studied for 30 years, if you have 100% primary balance, the up going mass of the crank, conrod, piston assy is matched by an equal down going balance weight mass.
However, the bad news is that at any point other than straight up or down, the balance mass is flying out sideways whereas the pison and most of teh con-rod are moving vertically resulting in major grief, secondary imbalance. With multi cylinder engines you can mitigate the effect a lot by balancing one cylinder's secondary against another's 180 deg apart. This is still not by any means perfect since the two secondaries are not acting at the same point so you have a couple which also rotates, generally flailing the whole thing about.
Vee configuraion engines can capitalise on having short cranks whcih helps and also using secondary balance from more than one cylinder to balance primary balance of another.
If you want to get into it in more detail, I'll try to dig up some reading.... hope you enjoy trig.
Richard
Navigation
[0] Message Index
[#] Next page
[*] Previous page
Go to full version