OutdoorKing Repair Forum Forums Restoring, Customising and Modifying Tech Talk SB Model 45 - Cutter Clutch Balance Device

Looks like I may have sparked some debate....It doesn't take a rocketologist to see that there is no way known in basic law physics that the clutch can be perfectly balanced with the cut out and bolts on one side. It makes sense to counter weight!
What really bugs me is the miss alignment from engine drive shaft to gear drive shaft...has anyone else noticed this ? They are way off being horizontal to one another!
My mate has modelled the entire SB45 and it's way off.....surely this causes vibration issues. Sorry but we work to 3 decimal places and little things like this mess with our heads.
Cheers

Hi Peettee,

I know where your coming from in regard to tolerances, but you must remember that this is a domestic lawn mower we are discussing here...The Scott Bonnar model 45, produced in 3 sizes...14", 17" and 20" some with different engine requirements and makes (ie. Kirby and Briggs) the height of the PTO shaft might have varied some.

To compensate for this, the bearing prior to the 12 tooth primary chain sprocket, is a double row self aligning type, which works very well in this configuration.

Re: debate on the counter-weight device, well the jury's still out on that one, we are waiting for members to post of their experience in the field.
All good stuff, mate!

If the engine's output shaft (PTO) is not exactly in line with the center of the sprocket bearing, there is going to be a problem. Yes, the sprocket bearing is self-aligning, but the crankshaft main bearing isn't, and the clutch-halves are rigidly bolted together and cannot act as a universal joint. If there is a basic misalignment of the two ends of that shaft, the machine will adapt to the situation by deforming the engine mounting rails at their least-rigid point, which is the inboard engine mounting bolt-holes. That happens to be right where the rails crack. Over time, the bore of the engine-side clutch-half will tend to wear and become loose, which may reduce the deformation and stress of the frame rail. Unfortunately if that clutch-half happens to be a setscrew type, this loosening probably won't occur and the rail cracking may happen sooner than it would with a captive cotter type, which seems to be rather prone to becoming loose on the crankshaft over time.

I think we need to hear a lot more about this reported misalignment of crankshaft with sprocket bearing. If it is a common problem it would not be hard to fix, but it would be better to do this before the rails crack.

My feelings on this issue Grumpy is:

If Peettee's modelling of the SB Model 45 is correct, and that all the different engines specs for the height of the PTO's, (as taken from the bottom of the engine and from the engine deck to the exact centre of the engine shaft) are exactly the same dimensions; (ie. for Kirby Tecumseh, Briggs and lately Honda and Chonda) then all Scotty 45 (twin rail) and Rover (all twin rail) would have flogged out inner clutch halves and cracked rails....
In fact, some are all original, no cracks and serviceable clutch halves.

Also, because of the technical nature of this thread, I would suggest we move it to Tech Talk...What are your thoughts?

Happy to move it to Tech Talk, Deejay - in fact if the thread continues, it needs to be there. I'll move it when I post this.

I agree that the popular engines in the time frame we are talking about were nominally made to standard dimensions. So, the engine output shaft alignment should always be the same, provided that the engines were accurately made, and that, on twin rail models, SB/Rover always installed the frame rails accurately then welded them in place without distortion. On single deck models, the proviso is that the decks and side plates were made accurately and consistently.

Aside from those possibilities of inaccurate positioning of the deck or rails, the other possible source of misalignment is that the mounting on the side plate for the self-aligning bearing could have either varied in position or been consistently wrong if the jig was made badly or designed badly.

So, if we are seeing inconsistency rather than consistent inaccuracy, that does not mean the problem wasn't misalignment - it just rules out consistent misalignment, such as might come from incorrect jigs for welding the rails or drilling the side plates. Inconsistent misalignment, which could be caused by sloppy operators in the factory, or bad maintenance of jigs, could still cause the crankshaft not to be pointed straight at the self-aligning bearing. However such misalignment happens, if we can establish that it sometimes does happen, and it tends to be present on mowers with cracked rails and/or loose clutch-halves, we might be a long way closer to clearing up some mysteries.

SB45 owners could check for misalignment (whatever its cause) fairly easily without needing advanced measuring equipment. I think all they need to do is remove the primary chain from the engine-shaft sprocket, disconnect the mounting of the self-aligning bearing so the shaft is supported only by the crankshaft via the clutch, and then see if the bearing's screw-holes remain in alignment with the side plate. To check for run-out as well, they can slowly rotate the engine's crankshaft (spark plug disconnected of course) and see if the bearing moves relative to the side plate.

I think perhaps we have lost sight of the actual set up of the complete assembly Grumpy.

The inner clutch half is secured to the engines PTO with a grub screw directly on to the PTO and the other securing the key in its key-way. It also contains the thrust bearing.

The other clutch half containing the clutch cork, spring and clutch cone is secured by the 3 retaining screws placed equidistantly around its circumference.

The clutch cone slides on the engine cutter shaft with a key-way, and a woodruff key inserted in the shaft itself.
That shaft is fully floating and only held in position by the clutch cone and cork in the outer clutch half.
Any wear experienced (or misalignment) of the cork could cause probs and in a way is (by virtue of its design) a contributing factor in misalignment issues.
Just food for thought and all good stuff!

So the clutch can act as a universal joint in the shaft, by working the cone against the cork lining? That implies that when the clutch is disengaged, the thrust bearing supports the cutter side of the clutch?

That's correct weight mate!
Don't forget there is the spring in there that engages the thrust bearing and the inside face of the cone, and that for the clutch to disengage, it only moves about 1/8". There is a considerable gap between the PTO shaft and the engine cutter shaft when fully assembled.
I have always suspected that the clutch design and the self aligning bearing act together as a sort of universal joint.

It does sound that way Deejay. Because the self-aligning bearing at the sprocket end acts as a non-driving universal joint, the cutter shaft is effectively like a car-type propeller shaft, and misalignment of the two ends (within reason) will not induce stresses in the PTO or the rails. So, alignment errors seem to be off the agenda. I'd still like to know whether the Briggs engines vibrate more than other engines though. One factor that might be relevant is that the engine-mounted-on-the-frame-rails might have a resonant frequency within the engine's operating speed range. While all of the relevant engines (Kirby Lauson, and Briggs 8 or 9 cubic inch) should, and probably do, have fairly similar unbalanced forces (see post on half-balancing in another thread) but different engine weights, the resonant frequency will vary from engine to engine. Resonant frequency is a function of spring stiffness and the mass of the vibrating object. The Kirby Lauson is probably rather heavier than the Briggs, so conceivably the Kirby Lauson has no resonant frequency within the common operating range, but the Briggs does. Measuring the amplitude of mechanical vibrations is feasible without special equipment, if anyone wants to do it.

Barely relevant story: I once found an engine speed at which the throttle return spring resonated in a fairly popular car. (You could feel the vibration through the accelerator pedal.) To find out whether it mattered, I made a point of driving the (new) car at that engine speed on the freeway on the way home from work, and on the way to work the next morning. The spring broke on the second day. I had it replaced and broke the replacement in another two days. Then I stopped doing it. Driving on a freeway at peak hour with no throttle return spring isn't fun after the first couple of times you do it. It wasn't dangerous (there was a rather light secondary spring in the system, as there usually is on car throttles) but there have to be limits to what you do in heavy traffic.