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

Hi to all ODK members,
One of new members, Peettee, has kindly sent me a schematic and a pic of what appears to be a balancing device (counter weight) fitted to a Scott Bonnar Model 45 (late model) cutter clutch and given me permission to post them here... The schematic looks quite an official document. The device in question is Item 28 in the schematic...

I have never seen one fitted to a SB 45, Rover 45, Protea or Star mowers (the latter 2, are knock-offs of the Model 45).
Here are the pics...




My questions are...would it work? Is it even necessary?
Could it cause vibration issues and damage the machine engine deck rails?
Comment please.....

It looks as if it may only be about 2 mm thick, so I doubt it will create enough imbalance to damage the engine.

The question seems to be, what would cause an imbalance that would require correction by that weight? Two possibilities come to mind.

The first possibility, is the problem associated with failure of the captive cotter. That seems to result eventually in the whole clutch running out of true, after the engine-side key gets mixed up in the thrust bearing and levers things about, creating clearance of the left side clutch half on the engine shaft. The resulting runout of the clutch seems to be the source of the vibration that breaks the engine mounting deck. We haven't proved that, but it seems a plausible explanation on the evidence we've seen so far. What we need is more cases and more experiments with partial cures. Now, perhaps someone noted the vibration that is caused by this issue, and thought a counterweight might alleviate the problem.

The other obvious possibility is that some of the clutches were out of balance. That could happen if the two halves did not mate perfectly where they clamp up against each other. Because there is a self-aligning bearing at the chaincase end of the clutch shaft, and the engine mounting deck is fairly flexible, at least on the twin rail versions, any slight mismatch of the mating faces of the clutch would cause the whole rotating system (two shafts and a large clutch in the middle) to become V-shaped instead of a straight line. A rotating imbalance of the whole clutch would result. That would cause a noticeable vibration. If that were a known problem, there might have been a counterweight available as a service fix for the vibration.

These are just guesses, Deejay. There are still plenty of mysteries to solve regarding that clutch, its shafts and keys, and vibrations in the machine.

I wonder if it's to do with a particular model engine being out of balance, perhaps they thought an engine vibration was the cause of the rails cracking and tyres to solve it? I say this because it is on the engine side of the clutch so has to be fixing something that is an issue even when the clutch isn't engauged

Mike, it is pretty unlikely a serious mass-production engine from a major manufacturer will be out of balance, unless it is the one-in-a-hundred-thousand that had a defective part installed accidentally. Also, remember that the clutch only disengages the clutch shaft and sprocket: both sides of the whole clutch are bolted together, and rotate whenever the engine rotates.

I don't know if you are interested in this, but single cylinder engines, including motorcycle engines, have a reciprocating weight (the piston, and the upper part of the connecting rod) going up and down rather than round and round. This causes engine vibration. On large, low speed engines that don't have to be light, it is possible to use balancing systems such as the Briggs Synchrobalance (a largish weight in the sump moving out and down out-of-phase with the piston), or a balance shaft rotating in the opposite direction from the crankshaft. For most engines though, you just put up with the vibration. Side-by-side twin 4 strokes have exactly the same amount of unbalance as singles. Horizontally opposed 4 stroke twins do not have the problem - they have a pair of pistons exactly out of phase with each other. This is the reason the old Briggs horizontally opposed side valve twins were so well-regarded by ride-on owners.

There is something that engine manufacturers can do to reduce the single cylinder vibration. They can increase the size of the counterweights on the crankshaft by half the weight of the piston and upper part of the rod. This halves the vertical vibration, but adds a horizontal vibration of half the magnitude of the original vertical one. The technique is called "half balancing" for obvious reasons, and it is practised on all but the most rudimentary of single cylinder engines.

So, SB45s, and single cylinder walk-behind mowers of every type, have engine vibration. The decks have to be designed to be able to stand the resulting loads. However if the clutch on the output shaft is unbalanced as well, the loads go beyond those the mower manufacturer allowed for.

It could be there to counter balance the 2 lock screws that secure the clutch to the engine, the counter weight doesnt appear on the parts list i have for the model 45

I suppose that possibility could be tested pretty easily. Is the weight of the counterweight equal to the weight of the two set screws? The rotational timing would be wrong, and I think the counterweight is probably considerably heavier than the screws.

The counterweight mounts exactly opposite the keyway, and the set screw that keeps the key from moving. This seems to fit the idea that its purpose is to balance out some kind of effect of the key and its set screw. Hence the speculation that it is to compensate for clutch bore damage caused by the key fouling on the thrust bearing.

Seeing that all the replacement inner clutch halves are made after-market without the balance device, I wonder who made the one fitted to this machine?
Could it be 'tenants improvements'?

It's shown in the special published IPL you posted earlier in this thread, Deejay. It can't be a tenant's improvement. Perhaps the answer can be found by investigating where that special IPL came from.

Hi Grumpy,
I hope that Peettee may know the answer to that...it would be nice to know if it is a Western Australian development.

I'm in Western Australia and I have a balance weight right here in front of me. A local mower parts shop sells them.

Total Weight: 0.055KG

I haven't fitted it yet but if I have time on the weekend I'll try and run the mower with on and see if there's any noticeable difference.

Hi dansblackcat,

Thanks for that info Dan,
As you would know from this thread, this device is an after-market development and was never fitted by Scott Bonnar nor by Rover Mowers.
I suspect it was a Western Australian invention to sort a problem with out-of -balance cutter clutches.

However, if your original cutter clutch is in balance, fitting one of these would throw it out-of -balance. Not a good thing, especially on a twin rail Scotty.

This is an interesting read. Im in WA and my SB45 14" has one of these and so does my fathers 17". Both are pre 1980 models and I have always thought it was a standard part. I know my local mower shop sells them because a few years ago I lost it when putting a new clutch pad in.

I haven't removed it to see if it causes a vibration.

Maybe WA was supplied all the dodgy out of balanced ones.

I've just done some more digging and found a website that sells these clutch counterweights.

It states the OEM Number is A20016. Not sure if this helps.

http://www.hwlr.com.au/index.php/rover-scott-bonnar-45

Hi Corduroy,
This thread has really got me tossed...why have we not heard of this device or have seen it, in the Eastern States?

If they are stating OEM (which stands for Original Equipment Manufacturer) and AFAIK I have not seen a parts list or IPB (Illustrated Parts Bulletin) issued by Scott Bonnar nor Rover (Australia) with this particular part or part number listed.
Here's a excerpt pic of the SB Model 45 parts list:

This is a real mystery.

Hi Deejay,

Agree, very strange. I have done some more digging. A guy at my work use to work for MEY in Perth. He was the production manager for around 10 years. He said the SB 45's that arrived from the wholesaler, some (not all) had these counter weighs on them and would always advise the buyer not to remove them because it would cause vibrational issues.

Its probably looking likely that WA did receive a bad batch of clutch housings.

Hi Corduroy,

I have spoken to Bruce, (forum administrator and site owner) about this issue...He said he will make some inquiries about this balance device from some friends in the business, and that may throw some light on the matter.
we shall have to wait and see.

Hi to all,

Below is the information on the model 45 technical information relating to the clutch counter weight.

Hi Bruce, and ODK members,

Thanks for making the inquiries and finding this information for us... and Rover knew about this issue in 1991!

I didn't believe it could have been caused by a well used Briggs engine, but there you go....a Model 80202 is at the root of the problem and the fix is the new style inner clutch half (engine side) with 2 longer screws and the counter-weight.

I see you have the counter-weights already listed in the Online Store...anyone with a new clutch half and that model Briggs will perhaps need one, plus the replacement screws, if suffering vibration issues.

Well done mate, and thanks again for this info.

We need to find out more about the whole clutch bore and captive cotter saga.

We know that quite a lot of SB45s with twin-rail decks, have cracked either the front or both rails, at the inboard engine mounting points. We know that a fair number of SB45s with captive cotter engine-side clutch halves, have had the bore of the engine-side clutch half wear and come loose, after long service. We now know that Rover's service people believed that if you used a set-screw engine side clutch half (the type that replaced the captive cotter type after the Rover take-over of SB) together with a fairly tired 8 cubic inch Briggs engine, there was excessive engine vibration.

There are more than enough SB45s owned by Outdoorking members to solve this issue properly, if we collectively want to do it. There are some basic questions that members will already have information on - we just need them to post it, preferably in this thread.

Question 1: Do all of the SB45s that have cracked their frame rails, have 8 cubic inch Briggs engines (mostly 80202, but perhaps 80102 and 81202 models as well)? Does anyone have a Kirby Lauson SB45 with a cracked frame rail?

Question 2: Do all of the SB45s that have cracked their frame rails have captive cotter engine-side clutch halves?

Question 3: Do people with SB45s with 8 cubic inch Briggs engines, find that the engine vibration gets worse when they replace the captive cotter engine-side clutch half with a set-screw clutch half?

If we can get answers to those questions, and they point to the 8 cubic inch Briggs engine being a common factor in pretty much all of the cracked frame rails, we could confirm the diagnosis with a fairly simple test. Someone could just get a Briggs 80202, sit it on a concrete floor, and fire it up. If it pogoed around the floor like some of the old British engines did, that would tell us something. If he or she then repeated the test with another engine (say a Honda GX120 or a 6 cubic inch Briggs) and found the other engine didn't pogo, we would be close to being able to say the 8 cubic inch Briggs looks like a strong suspect for the underlying cause of all of the SB45 vibration, frame rail failure, and clutch bore failure problems.

Gentlemen, I may be able to add some info. This afternoon I stripped a Rover 45 with badly cracked front and rear rails (the front had broken through). Everything appeared to be factory original. The mower was fitted with a Briggs 91202 manufactured in 1993.

Here is a photo of the clutch, once removed. Obviously it is missing a nut, which doesn't help, but the counterweight is present.



So I checked my Rover daily driver, also a 91202 from 1993 and the counterweight is present. Thankfully the chassis cracks are not