The BSA A7-A10 Forum
Technical (Descriptive Topic Titles - Stay on Topic) => Gearbox, Clutch, Primary => Topic started by: Swarfcut on 11.09. 2021 18:24
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There's a couple of folks with problems concerning the Cush Drive. While in principle the operation is the same, the design changed over the model years and the parts do not interchange without modification.
The S/A Drive sleeve originally had a machined portion inboard of the sprocket support flange to run on the oilseal. This part was changed to a separate spacer (67 1138) and drive sleeve (42 0069). For 1955 only, drive sleeve 67 1134 is also listed and these latter two parts look the same, but may differ slightly in the width of the sprocket support flange/collar.
The S/A crank nut is flat faced and bears directly on the end of the splined drive sleeve when the nut is torqued to the accepted 65 Ft Lbs.
A S/A Drive Sleeve to hand measures 51mm long, with a support flange/collar width of 5mm. Loading on a sprocket and the sliding cush member leaves 8.5mm of the sleeve exposed. Placing the nut onto the sleeve acts as a stop for the sliding member, so here the maximum lift on the cush is 8.5mm. The spring is 3 1/4 turns of 7.8mm wire (Vernier)
In this case the space for the assembled spring is 32.5mm, being the distance between the flanges on the cush sliding member and the retaining nut with the cush settled and the nut tightened. Take the full cush lift from this dimension gives a possible compressed length of 24 mm for the spring... Coil bound at maximum lift as in Bag'o Nails earlier post. Add the thickness of the tab washer and it all just about fits.
These parts are from a 1956 Shooting Star, but should be the same ballpark dimensions for all models. So at least you have some comparison of what's out there. Always assuming my parts are correct....
The Plunger design has a longer drive sleeve, at 57mm, and with the Duplex Sprocket and sliding member in place, both are more or less level with the cush settled. The Plunger nut has an extended reduced diameter nose, and the sliding member passes over this as the cush works. Using a later S/A nut will effectively lock the cush drive solid, tightening down on the sleeve or sliding member, which ever is proud.
Available lift for the cush is the length of the extended nose on the nut in the case of the Plunger. For the S/A, it's the difference in length between the drive sleeve and the sliding member assembled with its chosen sprocket.
Swarfy
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Hi Swarfy and All
I have added a reply to Bagonails clutch adaptor thread with some measurements.
Somewhere around 55/56 BSA changed the primary drive components in order to make room for the enclosed chaincase.
The parts that changed to move the chain case and drive outwards were to the best of my knowledge the cush drive sleeve collar width and the mainshaft adaptor for the clutch, then the primary case was move outwards by adding the big circular spacer between the crankcase and inner primary case.
It would seem that you have the earlier sleeve with 5mm collar ? where the ones I have are all 8mm
I do not have a 55 parts book but am now wondering if there was a change in the spring length (number of turns) as a new spring I bought from Autocycle Eng. has 3 3/4 turns where as the others I have 3 1/4 turns
John
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New parts, old parts, wrong parts ... all part of the restoration tapestry.
Surely, in this case, the simple solution would be, regardless of whether parts are wrong / miss-matched or whatever, to turn up a spacer such that the spring is fully compressed when the cams are around 3/4 of their full lift?
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swarfy all these parts is a good reason why things get mixed up over the years for whatever reason. in my 54 to57 parts book two springs are listed for s/a model . 67-2062 for 1954 67-1136 for 55/56. the sleeves are 15-1555 1954. 67-1135 1955 42-0070 1956. the bearings which i sometimes call the sleeves are 67-1097 1954 67-1134 1955 42-0069 1956. now i understand why the chap in the shop i talked to in the 80's told me to go in his box and get what i wanted, and in those days just like now i hadn't got a clue *yeah* *beer* *lol*
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CJ. Yes, parts are from an early S/A bike, and to echo RD, a simple solution. The SRM type nut is a definite improvement, but as in degsy's recent example, not suitable for plunger models without a modification to allow the cush to lift and yet still tighten onto the drive sleeve.
Swarfy.
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I did up a little sketch showing my assembly dimensions (a picture's worth a thousand words, especially one with dimensions on) *smile*
Yes of course I can and probably will end up making a spacer and it is a simple solution. Not however, a perfect one as the cush movement will still be stopped by the spring binding rather than against the nut as would have been the original design intent. Whether this'll be an issue down the track remains to be seen but I guess I won't be the first to smash a cush spring.
As can be seen I have an excess of thread behind the nut and if I neglect the silly split pin I can fit a 5mm spacer in there and still have at least 2 threads showing outside the nut, clearance to the primary case remains to be determined.
Another possibility is grinding the ends of the spring. eg. if I can take 1.5mm off each end I can get back to spring just clear of binding at the limit of movement which would be preferable. I have a mate with a surface grinder so maybe we can...I can shave 1mm off the inside face of the nut as well which is much softer than the sleeve. This might make all the difference.
I suppose the main reason for raising this was my surprise as to after renewing almost all the parts with what I have taken as being the correct ones I still had a problem. Anyway hopefully this info will in whole or in part help someone else out one day.
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Hi Ian,
Looking at your drawing the "nut" rebate measures 11.3 much the same as the pattern one I photoed
https://www.a7a10.net/forum/index.php?topic=9646.0
The original I measured was 14.5mm , do you have an original one? as this would add 3.2mm to the space for the spring
I searched again today but cannot find the original or pattern nuts, but found another cush drive cam with an even wider shoulder, fitting this would limit the cam operation to app 3mm
The width of the cam shoulders does not affect the fitted length of the spring only the amount the cam can move before it hits the outer nut face
I think that the shoulder needs to stop the spring compression before the spring becomes coil bound ???
previous measurements, I found that the depth of 3 cush drive collar shoulders varied from 9.2mm , 9.5mm & 10mm
where this one is about 14mm.
Maybe the wide shoulder on this goes with the 5mm collar sleeve to limit the travel and prevent a problem Sav had *????*
Reply #8 on: 15.03. 2018 09:31 ยป
Just had this one myself, took the primary chain case off my 1961 A10SR to service and clean the clutch which has been dragging/slipping a little, only to find the cush drive jammed open with the tips of the lobes just engaged, more travel than the limit on the example here, just not quite enough to drop over to the next lobe but to get wedged at the top of the travel. The nut had not loosened off either and split pin was in place. SRM nut arrives this morning so should know more then.
*conf2*
John
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I think that you will find that 1954 had a four lobe cush.
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trev i know you are right about that my book shows the 4 lobes and is 1954.
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Ian. I've revisited the original post, the available space for the spring is the crux of the matter.
My drive sleeve overall length is near as dammit the same, at 51mm. But my sleeve only has a 5mm collar leaving some 46mm for the sprocket and sliding cush sleeve. From the diagram it looks as if the available cush lift here is a matter of only 3.2mm. My example has 8.5mm. In other words the drive sleeve you have is shorter than expected, or the sliding sleeve and sprocket combination is larger than usual. The depth of the spring collar on my sliding cush sleeve is 12.5mm, on the nut it's 11mm. I've got some 33mm between the collars to take the spring. These are all from a 1956 model.
I reckon the drive sleeve you have is not quite right, hence the excess crank thread and lack of space for the spring. Either as a mis machined rogue part or not the right part for use with the sprocket you have. All my sprockets have a barely raised boss to bear against the drive sleeve collar.
Your 37.4 mm dimension contrasts markedly with what I have (31mm) even after accounting for the differing drive sleeve collar widths, 8mm (yours) versus 5mm (mine). Here's my drive sleeve, sprocket and sliding sleeve for comparison. 8.5mm of drive sleeve showing, in effect the available lift on the cush ramps. Plus the backside of a couple of sprockets, showing minimal raised bosses of 0.5mm.
Swarfy
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Ask a silly question time!
Should the cush drive be able to travel over the top of the lobes when the spring is compressed to rest in the next depression? or is it just a shock absorber?
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Definately should not.
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Hi John and Swarfy, (and anyone else who's listening!)
Well between us we've probably got one of everything it sounds like, unfortunately my set is the least compatible it seems. Clearly there are a lot of different parts out there in the mix but I will have to try and make this thing work. I have two basic issues as I see it ( I have other issues but we won't go there)... *eek*
1) I only have a maximum of 3.2mm of cush travel. This is probably not enough for a cushy cush although I've not seen any specs or requirements as regards what the max and min travel should be but I am aiming for a minimum of 5mm, lets say 5 - 8mm would be good, anything more than this probably isn't necessary unless you are a very mechanically unsympathetic pilot.
2) I am coil bound on the spring as soon as I tighten the nut in fact I would say, this is the biggest problem as we really need the nut to tighten hard onto the end of the sleeve bearing as has been very well covered already and the cush is therefore solid with no travel available.
Further to the above whatever I do to increase the travel is no good unless the spring is able to freely compress. If not the spring becomes the limitation on travel when the coils bind and this is undesirable for spring life. It also means the spring is working at maximum even excessive preload and I have an idea this also makes the cush less cushy and reduces spring life.
The way I see this I need to tackle both these issues to obtain some cushtyness so I can increase the travel on the sliding sleeve by putting in a spacer between the nut and the sleeve bearing. Without the spring this will allow more travel and increase the space for the spring by the same amount, however, it wont stop the spring binding as soon as the gap between the shoulders gets back to 24.
ie. In my set up a 5mm spacer gives theoretical cush travel of 8.2 (3.2+5) and a spring gap of 29 (24 +5) but the spring will bind again after 5mm travel.
This means I need to find at least another 4mm clearance between the shoulders say to ensure the spring never binds or remove 4mm off the overall height of the spring, or a combination of both.
At this stage I am reviewing my options as follows:
Change the nut, well this was a new one bought to replace the one that came with, which had been ground down by 3mm on the end by the PO. (He was using an incorrect Plunger drive sleeve bearing with a 5mm shoulder which put the sprocket 3mm out of line with the clutch) No John I don't have a 14.3 nut but if I did I would certainly use it! I could buy another nut say the SRM one but this doesn't give me much improvement so I think I will stick with modifying the one I have. I can pinch 1mm safely here I reckon.
The sliding collar is definitely a problem as you point out Swarfy (thanks for all those measurements by the way very useful). If I could pinch a mm out of the collar I would but it is really hard, carbide doesn't touch it and hard to hold to. If I can find a way I will try to grind 2 off the nose and at least 1 off the shoulder but workholding is the challenge.
The sprocket is a new 22 tooth from Feked and seems correct, at least similar to John's examples with a 1.6mm raised shoulder on the rear. I could grind this down but anything removed here puts the sprocket out of line with the clutch again...
So the only other option I'm left with as I see it is to grind the spring down. I bought a new spring as well which has 3 1/4 turns but is poorly made and the ends are nowhere near parrallel Also it is smaller on diameter and doesn't quite fit over my sliding collar. My previous spring is much better quality and although 3 1/2 turns squashes to 24 rather than 26! I intend to 'work' this to reduce its compressed length and reduce the likelyhood of binding up in use. (Grind the end faces down 2mm each end and cut the tails back a little, all to add extra clearance. *work*
Then mostly I am going down the pub! *beer*
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Ian. That use of a plunger drive sleeve is interesting. To be clear I'm talking here of part 67 2071. This fits the crank, same internal splines, etc, but the OD where the duplex sprocket runs is different, a gnat's knacker under 36mm, but for the S/A sleeve I have it's 35mm. So to fit the S/A sprocket it must have already been ground down. The depth of the internal splines on the sliding sleeve is also different, S/A splines are marginally shallower, but will fit the Plunger Drive Sleeve. The amount of load bearing area is reduced, so no heroics, just in case.
If possible using this existing longer Plunger sleeve instead of the correct part would gain the missing mm's (there's around 5mm difference in the two parts). With luck the chain alignment will be too near the crankcase, shims 67 2056/7/8 between the drive sleeve and oilseal spacer 67 1138 will fix this. Not ideal, but at least uses most of what you already have. A Plunger Drive Sleeve to hand has a 6mm collar, under the 8mm one you currently use. The root cause of this conundrum remains. Depth of cush ramps on the Feked sprocket look a little shallower than mine. Could this be it? I've got 9 and 9.5ml on my sprockets, measured from lowest point in the cush ramps to the boss on the back.
Swarfy.
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Hi Ian
I think Degsy's posts have cracked the problem,, https://www.a7a10.net/forum/index.php?topic=16565.0
This type nut would allow room for the spring 67-2054 *????*
https://www.ebay.co.uk/itm/254121076906
John
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Hi Swarfy, See photos below for what I called the plunger sleeve, I'm not certain exactly what it is but it bears comparison to the part you refer to. I'm calling it Orig. as it is the part that came with the bike. Yes it fits fine on the crank but the 5mm shoulder put the sprocket 3mm offset inboard to the clutch. I guess I could space it out again but I'd be back at 8mm off or the same as my new sleeve. In addition as you have stated the splines are different with much less contact, this is something I had not noticed so thanks for mentioning! I also show the diameter differences which on my parts here are not great but the new part is a slightly closer fit to the sprocket and of course lacks the undercut so does support the sprocket better, to keep things running true. For my money I'd rather stick with the new part I think. On to the sprockets my original sprocket measures 10.2 at the minimum of ramp to the rear face so slightly fatter than yours and the new one is fatter still at 11.6 so this part is also in the mix for sure. If I had your smallest sprocket that would give me 2.6mm straight off!
The parts I have now though are what I have to make work so I think that's about it really. Quite amazing the number of parts the same only slightly different and also the variation between parts that are supposed to be the same...whatever happened to standardisation and interchangeability of parts then?!
Hi John, thanks for this idea, it is something I could consider for sure. I wonder if anyone has any dimensions for this part?
It looks like it might have a longer threaded section, clearly would allow more movement of the cush sliding sleeve and potentially more room for the spring too. I missed the Degsy thread as I hate to say I am well behind on the forum having been away with the wife for a vacation for the past week. We are very lucky here in SA being free of the virus currently so we were able to get away up to the NT for some Croc spotting and very careful swimming in warm springs which was nice. 38c was a bit of a shock after leaving here in the morning at 3c. What a difference about 3000km can make.
I'll add some more photos in a following post.
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More pics, titles on the photos should be self evident.
Difference between the old and new sleeve - fit on splines to the sliding sleeve.
Slight difference in diameter where the sprocket runs
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Ian. The drive sleeve with the undercut is indeed the plunger version. Using a plunger nut with your S/A drive sleeve is fine in theory, but as you appreciate with the plunger design the slide moves from a splined to a plain section. While the ID of the drive sleeve splines and OD of the nut nose are the same, the internal splines on the S/A cush are shallower as shown, and will not bear onto the plain nose of the Plunger nut. The S/A slider is also of less depth than the Plunger in comparison and there may be a tendency for it to cant over and jam at full lift.
As the plunger sleeve has already been ground down to take the S/A Sprocket, a complete plunger cush set up could be used, although the profiles of the cush ramps are a poor match. From your picture the Plunger Sleeve looks to offer enough lift, so should work with the rest of the bits. The poor depth of spline engagement is the stumbling block here, but is obviously enough to take the load as in the later design.
The reason for the small amount of lift using your "correct parts" still baffles me. Somewhere out there is there a S/A drive sleeve that pokes at least 8mm above your stacked sprocket and sliding sleeve? The last paragraph of the original post to this thread says it all.
Chain alignment is taken care of by shimming between the drive sleeve and oilseal spacer, as already mentioned and has no influence on the amount of available cush lift, affecting only the final position of the tightened nut on the crank.
Swarfy.
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Yep all valid comment Swarfy and much appreciated.
Somewhere out there is there a S/A drive sleeve that pokes at least 8mm above your stacked sprocket and sliding sleeve? The last paragraph of the original post to this thread says it all.
tell you what when I find one of these spurious sleeves I shall be fitting it but until then...
Chain alignment is taken care of by shimming between the drive sleeve and oilseal spacer, as already mentioned and has no influence on the amount of available cush lift, affecting only the final position of the tightened nut on the crank.
and therein lies my problem. While I could theoretically fit the plunger sleeve as before, notwithstanding all the other issues we've been discussing; by shimming behind it to move the sprocket back into line I am also running out of threads on the crankshaft so really I may as well fit a spacer ring, turned up to be a good close fit on the crankshaft and with an O/D just under the diameter of the splines, to allow the cush sleeve free movement. Tomorrow I am going to encase my spring inside a thin wall tube with a slit in it so I can chuck it in the lathe and try to turn the ends of the spring back as it feels tough but not too hard. So we'll see how this goes...My fallback plan is to use the ends of the tube as a guide to keep things square and hand grind the end faces! One way or the other the spring is losing 4mm in length *ex*
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Hi Ian,
I think you are worrying too much about the required space needed for the spring?
On my A10, there is app 30 mm for the spring, that leaves 4-5mm for the spring to compress before coil bind, at more or less the same compression the sliding cam collar will meet the nut,
Its been like that for a fair few miles and has not caused any problems
John
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Ian, Mikes in Qld has several sleeves of different types. Here is the correct one for the 1960 Flash
https://mikesclassiccyclespares.com/product/42-0269/
Col
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G'day John,
Hi Ian,
I think you are worrying too much about the required space needed for the spring?
On my A10, there is app 30 mm for the spring, that leaves 4-5mm for the spring to compress before coil bind, at more or less the same compression the sliding cam collar will meet the nut,
Its been like that for a fair few miles and has not caused any problems
John
Well really that is the crux of the whole thing John if you check back in my sketch (previous post above). The spring is coil bound at 24mm (squeezed in a vice) and I only have 24.1mm for the spring when the nut is tight against the sleeve bearing. So basically when the nut is tight my sliding sleeve can't move, I have no cush! In addition even with no spring I would only have 3.2mm available movement. Hence all the discussion about the various options. I'm literally now going to fit a 4mm spacer and cut/ grind the spring end faces to reduce the coil bound length to ensure I can get 6-8mm travel without binding on the spring.
Hi Col, That is the correct part I am using now. As you may recall the original incorrect (plunger) part pictured above was the reason for my primary drive being 3mm misaligned. I cured one problem only to cause another. I've also bought a new spring and nut. The issue seems to be either the sprocket (also new) or the sliding sleeve or both but I've had enough now after waiting weeks for parts and finding them wrong or incompatible somehow I am going to have to make it all work together! *pull hair out* *work* *eek*
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Been a useful thread on the mysteries of the cush drive thanks guys.
My A10SR had the cush drive and rubbers in the clutch mounting. When I took it apart to replace the ailing clutch I found the cush drive spring bound up on the lobe.
Solid new clutch basket and a shim to stop the cush travel enough for binding, always wondered if I had got it right and whether the lobe should be able to pop over the top in case of seizure.
Understand it all now ( i hope) thanks!
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Hi Sav, good to hear you have got something out of this if only to confirm you made the right decision to go with a cush in the clutch, or maybe you have both, some do I have read.
I am not familiar with the detail of your clutch but I can't imagine any circumstance where it would be desirable for the cush lobes to ride over each other and reset, even in the event of a seizure to be honest, you'd be better off whipping the clutch in I would think. I think the intention would always be to limit the extreme of cush movement to a hard stop well before that could happen in the interests of road safety and maintenance of clean undies! *eek*
Happy and event free travels to you.
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... in the interests of road safety and maintenance of clean undies! *eek*
😳