Back Gear selection on smart and brown lathe When I momodelA mark 1

[john walker 5Participant @johnwalker5]

With the lathe stopped I engage The out position on the back gear selector lever but when I then try and restart the machine the spindle won’t turn. Return the lever to the in position and the spindle  turns . I don’t know what to do I am a novice and any help would be appreciated

Thank You

John W 

[john walker 5Participant @johnwalker5]

[John OlsenParticipant @johnolsen79199]

Usually with a back gear there are two things to do to engage it.

 

One is that you move the actual gears into engagement…that will usually be something like a lever with two positions, and will be the one you have been trying.

 

The second thing is that you must disengage the direct drive on the spindle from the drive pulley to the spindle. I don’t know the Brown and Smart, but for example on my Myford ML7 there is a small allan screw on the bull gear of the back gear. That screw has to be loosened and moved radially outward. That disengages a little drive dog between that gear and a smaller gear that is attached to the pulley. So the pulley turns on the spindle, drives the small gear, that drives a large gear on the backgear shaft, the small gear on the backgear shaft drives the bull gear on the main spindle, and the main spindle goes around slower than normal. Depending on accessability, you might be able to see how it is done on your lathe.

 

If you don’t disengage the normal drive on the spindle, when you engage backgear it will lock the spindle. This is sometimes used as a way of locking the spindle, but be cautious with it as using it to get a stuck chuck off will put a very big load on the gears.

 

Hope this helps.

 

regards

John

[Dinosaur EngineerParticipant @dinosaurengineer]

I strongly recommend that you join the “smart_and_brown_lathes” group on “Yahoo! groups”. There are lathe manuals, operating instructions, etc for most Smart & Brown lathes on this site. I’m not familar with the model A but do have a S and B 1024 roundhead. There will be group members who have Model “A”s who I’m sure will be happy to help.

Good choice of lathe ! 

[john walker 5Participant @johnwalker5]

Thank You I’ve got it sorted now

John Walker 

[Chris CooperParticipant @chriscooper5]

To bump a very old thread, what did you do to sort this problem?

I also have a Smart and Brown Model A and the same thing happens. Engage the back gear and it won’t turn. Clearly there is a second step, but I don’t know what it is.

[BazyleParticipant @bazyle]

You are talking about the big industrial lathe not the small one (Sable) about the size of a Boxford? https://www.lathes.co.uk/smartbrown/page2.html

Lift the little hatch on the top of the headstock where you get at the belts. and you can see the bull wheel (big gear) There is a pin to pull out or unscrew on the face of the gear. Best I can remember from 1978.
For a while we had access to one at work until safety crap banned us electronics engineers from mechanical things except the drill. I started my V10 on that lathe following the booklet which said mount the cylinder on the faceplate – it disappeared down the huge hole – instructions were intended for small hobby lathes.

Beware the lever on the front bottom right of the apron also turns motor on aswell as the push button contactor.

[old martParticipant @oldmart]

Just noticed this thread about the model A Mk1, and the backgear is the same on the Mk1.5 that we have at the museum. Lucky people have a Mk2 which has a completely revised and simplified back gear mechanism.

Look on the left side of the belt pulley and you will see a hole for a hexagon Allen key, I think it is either 7/32 or 1/4. The key has to be shortened because of the narrow gap it has to fit in. The shaft that the key fits in can be turned which cams out the coupling pin so the belt pulleys are no longer driving the spindle directly. Then the backgear can be meshed with the bronze gear which lies hidden under the cover on the right hand side which is held down by two 2BA shcs. Never abuse the backgear by meshing while running the lathe or you risk stripping all the teeth off the bronze gear, just get into mesh by turning the chuck gently by hand as the lever is turned.

I can remove the spindle from a model A in 15 minutes and know them quite well.

[Martin ConnellyParticipant @martinconnelly55370]

Just to follow on from Old Mart, the hex key only has to go half a turn to engage or disengage the lock. When disengaged and back gear not selected you should be able to turn the spindle without the pulley turning. Make a full turn to be sure the pin is fully retracted. When you engage it after you no longer want back gear turn the spindle or pulley to re-engage the locking pin before using the lathe again, you should hear and feel the pulley and spindle lock together. Failure to do this may cause the pulley to spin up and then damage the pin as it lines up with, and tries to drop into, its locking location.

Martin C

[old martParticipant @oldmart]

I have made a spare locking pin for the lathe which has not yet been required. The lock hole in the steel ring inside the large end of the pulley was worn and bellmouthed from impropper usage so I turned the disc round and made a new hole in the opposite side, as good as new. That disc is keyed to the spindle and held tight by a threaded lock ring. That was about 9 years ago. Not all of Smart & Brown’s designs were very good. Unfortunately the drawings of the back gear mechanism are pretty well impossible to understand without actually taking the thing to bits, so its a waste of time my posting the parts pictures.

[Chris CooperParticipant @chriscooper5]

Got it! All sorted now thanks.

[Author]

Posts