Taper bushes and Keyways

[Roger B]

I am struggling to keep the flywheel on my diesel in place. It is 80mm diameter and is currently fixed to the 12mm diameter crankshaft with a tapered bush. This is ~16mm diameter at the large end and ~14mm diameter at the small end. It is clamped by 3 M3 grub screws (based on the Fenner taperlock design). In the last short trial it slipped at least 90° (or 90 + X x 360°). I am going to increase the size of the bush to 18mm diameter with M4 grub screws and am wondering about adding a 3mm key. This obviously can't be along the slit in the bush or opposite it (or can it be opposite??). This leaves two places, next to the slit or away from the slit. Any thoughts    ?

I have similar problems with the starter dog slipping but I can see a way to redesign that to increase the clamping force. I would prefer to avoid keying the starter dog as I had a previous engine sieze whilst being turned with a fairly powerful drill that didn't do it much good 

[Alyn Foundry]

Hi Roger.

Here's how they place the keyway commercially.

Cheers Graham.

[Jasonb]

With the shaft and bushings already so chewed up you are going to have a job to get things to bite unless you make a new shaft and bushes. Not a fan of them myself but rather than use grub screws make a flanged bush and pull it into the hub with cap head screws.

Personally I would taper the crankshaft and have a matching taper in the flywheel and a woodruff or plain key for good measure and a nut to pull the flywheel onto the shaft.

[Dave Otto]

Personally I would taper the crankshaft and have a matching taper in the flywheel and a woodruff or plain key for good measure and a nut to pull the flywheel onto the shaft.

Exactly the way the flywheels are secured to the crankshaft on my 1933 Deutz single cylinder Disel engine.

Dave

[derekwarner]

Roger....you appear to have a very high ratio of bush length to diameter [2:1?] which appears to provide a substantially higher ratio over the proprietary [Fenner] bushes

The image has me baffled ...the bush appears to be still engaged in the pulley, however a shear rotated portion remains on the engine shaft?. is this the Starter Dog section that has a de-coupled bore? 

As an apprentice 50+ years ago, I remember reading the paper pamphlet [that came with each new Fenner Bush box] espoused the fact that the bush material was a quality spheroidal Grey Iron.......[the Fenner product always had a light Powder Blue colour painted ends, which facilitated marking out for the drillings & key]...a Tradesman or Foreman would always check the marking out prior to my machining  ]

[this is absolutely the opposite function of a steel shaft rotating in a Grey Iron Bushing] as Iron [when forced] will provide a greater coefficient of friction over a steel bush on a steel shaft'

The sizing of bushes for a 'common shaft size' were in 2 catagories [OD and length] based on installation types or level of impulsiveness......[soft start - DOL/Diesel] 

Derek

[Roger B]

Thank you for your thoughts   

The crankshaft under the flywheel is not badly damaged. The section under the starter dog is cut up and has already been turned down to 11.5mm and sleeved once with the sleeve being fitted with Loctite 648. The sleeve subsequently came loose (which stopped my last trials  ).

[john mills]

Hi it is good to see the results you are getting
I have over the years used all of this types of taper fittings while taper lock are of iron i have made and replaced
and fitted all of them when they have failed some are more complex than others  and can have heavy well fitted
made to precise fits made of high tensile alloy steels and when overloaded i have seen them all fail when things go wrong metal working machines with jam ups can do amazing things.i see them all give long dependable service
when fitted properly.         some with heavy flanges pulled up with bolts and taped holes for forcing apart can give long trouble free life  but they still will 
fail too           John

[Roger B]

Thank you for your suggestions 

Making the bush out of cast iron is an interesting thought. The crankshaft is similar to EN8 (ST50) and the flywheel is some stainless like scrapbinium, possibly one of the mould steels.

The crankshaft is already a little small in diesel terms, it should be around 0.7 times the bore but is actually around 0.5 times, so I would be concerned about turning it down further to make a taper seating. That is something to consider if I build a mark 2.

It I make a new sleeve and Loctite it in place will it survive me cutting a keyway with a woodruff type cutter or will it be ripped off   

I intend to modify the starter dog by cutting off the 'spring' part and fixing it with two cap heads instead of one. This also will allow me to cut a keyway.

[john mills]

Hi is the sleeve where the taper fits?
i would not like to turn the shaft any smaller to make it tapered   what about make the tapered adaptor to fit the
shaft without the bush another part to slip .if you could cut a key that would help drive but not cut any more
than needed to make a key.    John

[MJM460]

Hi Roger, so disappointing to have such a failure when you were getting so close to success, but I suggest that that flywheel slipping is exactly what you do want to happen in such circumstances.  If it had not slipped, then the force which shortened that grub screw would have caused the next weakest link to fail.  Bent or broken crankshafts could be expected in such circumstances, bent conrods, broken pistons or even a broken crankcase.  I would suggest cleaning up the shaft and otherwise stay with your original excellent design.

As I see it, the law of conservation of angular momentum means that the rotational momentum for the flywheel was brought to zero in while the piston travelled the distance from the unreformed to the deformed dimension of the grub screw, which happened in a small part of a revolution.  I am assuming that the piston did not pass through top dead centre, as I assume the deformed length of the grub screw would not let the piston past top dead centre. 

Change of angular momentum per unit time tells you the torque involved with an easy calculation based on flywheel dimensions, grub screw change in length, rpm if the con rod geometry is used to estimate the amount of rotation while the grub screw is being compressed.  Certainly easier and probably more accurate than estimating the force from the grub screw change of length, but it would be interesting to compare the two calculations.

Similarly for the starter dog.  Having the engine seize while being turned by the starter is indeed unfortunate, but to cater for this possibility a shear pin, or similar weak point, would be desirable to prevent more serious damage.  I am more familiar with shear pins are used in the propellor drive for out board motors.  I have experienced breaking one of those, and it broke cleanly, was quickly replaced and the motor was going again.  Just as well I had those spare shear pins on board.  I believe that this would work just as well on your starter dog.

MJM460

[Roger B]

Thank you 

The flywheel moved in 'normal' running. I made a TDC mark with a marker pen when I put it back together after the jam up so I could check the injection timing. After the short runs in the video clip it had moved ~90° anticlockwise suggesting that it was slipping when the engine fired.
I will not cut keyways at this time, but the modified designs for the taper sleeve and starter dog will allow it if required later.