Drill with synchromesh????

[usenet]

See:-

drill supposedly has a "three speed synchromesh gearbox"! :-)

[usenet]

If you do a google search for this drill (Metabo Bst 12) lots of the descriptions claim it has a "three speed synchromesh gearbox". Curiouser and curiouser.

More to the point none of the sites seem to agree on its torque or the battery capacity.

[Tony Bryer]

It presumably means you can change gear while the motor is running .. if you want to.

[usenet]

I went and took a look at the Metabo site and they describe it as "3-level planetary gear" which makes a bit more sense.

[Christian McArdle]

I can change gear on my mains drill. It makes a nasty crack and I only do it when not concentrating. Unfortunately, it a cheap drill without rotor brake, so you have to wait 10 seconds normally, which is a PITA.

Christian.

[Dave Plowman (News)]

Yes. A planetary gear may be changed under full power with no distress - car autos have been doing this for years.

A car type synchromesh system wouldn't last five minutes without the clutch.

[Andy Dingley]

If you want to make a habit of it.

Easier, yes. But it doesn't help reliability.

There's a bronze ring with a cone clutch on it and tapered lead-ins for the splines on the sliding member.. When you try to engage the dogs, the ring gets there first. Once the cone has spun to the matching speed, it allows the dog splines to slide freely.

Do too much of it, you wear the nose off the dogs. then the blunt nose jams in the socket, locks solid, there's a bang and the ring breaks into fragments. You're now left with a rather sloppy crash box. On some gearboxes, the exploding clutch can get caught in the works on the way out and do more damage.

Porsche's gearbox design is different and doesn't use the same sort of bronze cone. They're more robust for being used clutchless. OTOH, I believe that if they break, they always lose that ratio.

[Dave Plowman (News)]

A crash box and a motorcycle type aren't the same. On a crash box the gears actually slide into mesh. A motorcycle box is constant mesh with dog clutches to engage them.

Synchromesh uses a friction clutch to 'spin' the gears to the correct speed to engage silently. But it's only designed to work on a box with the drive disengaged. To handle the engine's power, they'd be vast in size, and require considerable effort from the driver.

[Jan Wysocki]

Really? Do you mean you need a 'crash' gearbox to be able to do clutchless gearchanges like you can on a motorbike? I'd assumed that a synchromesh gearbox would make it even easier, but then I have no idea how synchromesh works. (I do know about epicyclic gear boxes.)

[Rory]

But it's only designed to work on a box with the

Do you mean drive disengaged, or just not while under power (ie with the throttle pedal backed off)? We have advanced driver training at work and the ex-police instructors (especially the older ones) often change gear without using the clutch. I do it myself when the mood takes me, and the higher gear changes seem effortless. Changing down is a bit more awkward (matching revs) but it's a potentially useful skill in case the clutch cable breaks.

[Andy Dingley]

That would be because "advanced drivers" are all too much a smartarse hazard for everyone else _ESPECIALLY_ older police drivers.

Why would you change gear in a modern gearbox without using the clutch? What advantage does this convey ? Make a habit of it on a V6 Carlton or some of the more powerful but weaker-boxed Sierras and you were just asking for trouble (and look at the scrap pile round the back of a police garage)

[Andy Dingley]

Just how old are you ? 8-)

There are plenty of "crash boxes" with dogs - pretty much anything post war. They're not sliding-gear boxes, they're not synchro boxes, but in the absence of a better term we call them crash boxes just the same.

Motorbikes have the advantage of a sequential shift too - they can shift from one ratio to the other with a lot less faffing about than a wobbling joystick.

What was the last crash box in production ? Landies ? bottom cog in Heralds ?

[Dave Plowman (News)]

If you match the gear speeds through careful use of the throttle etc, there's no need for any form of synchronizer. This was the basis of double declutching - needed with crash boxes.

However, if the speed isn't exactly matched, the synchromesh cones, which are a form of friction clutch - will try and do this, and if under load will either not succeed or be subjected to loads they're not designed for.

[Dave Plowman (News)]

On which car? I'm not familiar with commercial vehicle practice and was only referring to cars.

Rover for one made a constant mesh box with dog clutch engagement - pre WW2 - but it was hardly a common arrangement with cars. And it was mated to a freewheel which makes down shifts less of a problem.

All the cars I can think of - made up until the advent of all synchro boxes round about '70 - had a 'crash' first gear, where the gear teeth were normally out of engagement. And this practice continued much later with reverse gears.

There were a few earlier all synchro boxes on some cars, but these were usually three speed. Or not actually - commonly a four speed with first blanked off. Early Austin Healey and some Triumph models for example.

But they don't have synchromesh. A slight - or even not so slight - 'click' on engagement might well be acceptable on a bike, but not in a quiet car. There's also the question of feedback of this action through the lever.

Pure genuine crash box with no synchromesh at at all on any gear in a car? Something dating back to the early '30s, before Vauxhall introduced synchromesh in the UK.

[N. Thornton]

yup, you can drive cars clutchless in dire necessaity but you do have to get the revs dead on.

NT

[Capitol]

Not quite true. The gears are in constant mesh, but the one operating is selected by a clutch band within the gearbox. Also, the torque converter is a lossy liquid coupling which reduces the impact on the clutch bands and the drive train when the ratio is changed. The only common constantly variable drives( other than electric/electronic) are the twin pulley ones as pioneered in cars by DAF IIRC and now used in some stand drills. In cars I understand that these latter have an unequalled reputation for unreliability. I believe the Japanese may now have engineered a viable unit, but it's not on my acceptable list for a few years yet.

Regards Capitol

[Dave Plowman (News)]

Indeed. How else would you change gears with an epicyclic box?

But a TC isn't an essential with a conventional auto. Older designs used fluid flywheels which had near zero 'slip' above a certain speed.

Of course, most modern autos reduce the engine power momentarily on a change for reasons of smoothness.

CVTs. And they do seem to be unreliable.

Audi use a twin layshaft twin clutch synchromesh box where the changes are servo operated. By having adjacent gears on opposite layshafts it's possible to have a full power change because as one clutch disengages, the other takes up the load. Clever.

[G&M]

Only on changing down. And even then you can reduce wear to very low amounts if you listen carefully.

[Dave Plowman (News)]

Under full power? Not so. When changing gear without the clutch either up or down you remove the power near completely. If you didn't the revs would go sky high as soon as you got into neutral. And that's assuming you actually could under power - the gearchange would become very stiff.

An epicyclic auto changes gear by means of a clutch or brake band. These can handle the maximum power output of the engine.

[Andy Dingley]

There are plenty of car auto boxes that _can't_ handle this power on the brake bands. Look at the small BW boxes that popped up in big-engined UK cars through the '70s. For that sort of epicyclic, the ratio between max torque for constant running and max torque during a shift is quite high. They'd sit under your Rover or Jag quite happily cruising flat out down the motorway in D, but start forcing gear changes by holding it down in 1 or 2 with the loud pedal down, then slipping it up a notch manually, and they soon started to complain.

Epicyclic gearboxes in tools like electric drills often use dog clutches anyway.