Thanks 659, it was a small PSA actually, they kick up a hell of a kelly when reversing.
I always thought it was the indirect/reversing action causing the racket, but Lou insisted on knowing why the gear itself should make a noise, so thankyou for that.
As a minor aside, i seem to remember Rover 45's and similar having a particularly fragile reverse gear.
Was this true does anyone know, and if so what was so fragile about them, i've heard them making horrible noises in the past as if gear teeth were missing.
|
|
|
To add to 659's rather complete answer;
- the gear teeth themselves are also not ground to as good a finish or geometrical tolerance as those for the forward gears, particularly the higher ratios, and final drive
- some revers gear are formed on the outer of the 1st 2nd synchro hub, and so there's some movement under load, and the gears aren't running at any well defined centre distance
- for at least one of the three gears in the drive, one of them will typically end up being quite small in terms of pitch diameter, and hence will have a small number of teeth, and hence will either have significant undercut, or "correction", and so will be running in a less than ideal mesh
One interesting aside is that depending upon which side of the main mesh you put the reverse idler, you can obtain a resultant force which pushes the idler out away from the mainshaft and layshaft, or, pulls it closer in. The latter makes for a stronger gearbox.
Gear noise is one of those subjects which really intrigues me. There's a rare but interesting boo by Broersma, where he describes that the noise measured in ship's gears could enable the machine which originally cut the gear to be identified.
|
|
Individual ships (and other vessels) can be identified by gear mesh rates and other machinery rotational rates. Also used for machinery health monitoring.
|
One of my first cars was a SAAB 2 stroke (I wish...) which had a rather unusually configured 3 shaft box - in some ways like the present day Volvo transverse manual boxes. SAAB at the time bought gears from ENV.
My boss at the time used to work for ENV when SAAB first produced this gearbox. He described them as "sods, wanting Rolls Royce quality at Ford prices". Apparently, every so often a telex would arrive from Sweden - "the tool you are using to cut second gear has now worn by a further 1/10 thou - please change it".
659.
|
so often a telex would arrive from Sweden - "the tool you are using to cut second gear has now worn by a further 1/10 thou - please change it".
Modern CNC milling machines have a tool offset to counteract this. Hence a tool can be varied in size slightly and so long as the machine knows the exact tool size the end results are the same.
|
>>Modern CNC milling machines have a tool offset
For a purely generated surface, CNC offsetting works well. However, gear cutters produce the surface partly via a forming action, and if the form is worn, no CNC offset can correct for that.
|
|
|
|
Thanks NC, Lou would have enjoyed the conversations here, he would dearly liked to have read the book you refer to.
I assume then that quality cut gears with same size reverse idler wouldn't cause the whine.
Learn a lot here.
|
OK.
So why don't forward gears whine?
For idiots please.
|
Because they are helically cut, finely made, properly supported and optimally designed for quiet running.
To be a little more helpful, helical gears have the property of transferring the driving torque smoothly from tooth to tooth (ie a pair of teeth carrying the load will only cease to do so after the next engaging pair of teeth has begun to carry load, so that there is a continuous and uninterrupted transfer of torque).
Now, M. Citroen...
659.
|
|
|
|
>>I assume then that quality cut gears with same size reverse idler wouldn't cause the whine.
Not necessarily, it's just one factor.
The noise is caused by variations in the load on the gear during the mesh, and typically, there are a number of sources of this which all add up together.
In the ideal situation, the meshing involute teeth form a constant velocity relationship, and, in this ideal situation, there would be no noise.
To get some idea how small the imperfection needs to be in order to create noise, take a decent sized precision ball bearing, and roll it along a clean flat surface, and listen - even this degree of perfection makes a racket!
Even if the two meshing surfaces were perfectly formed, any variation in the material's stiffness from place to place, or from gear to gear will produce a tooth form error when loaded, and so, gears which are perfect when measured in an unloaded state might still make noise when loaded - the deflections of the gear case is also important here. I did some work on some TGV gear failures, and the deflections of the casing were an important factor in overloading the (already very highly loaded) gear and its shaft.
|
The noise is caused by variations in the load on the gear during the mesh
Yes i can see why the noise is created, very complicated stuff.
I suppose it's difficult to make such things tough enough whilst keeping undue noise to a minimum, whilst keeping weight down, and at the same time keeping the box easy and light to use, a heap of compromises.
|
>>Because they are helically cut
I presume that must put a 'thrust' on them.
How much does that cost in efficiency?
|
|
|
|
|
My contribution to this conversation pales into insignificance in the company of experts......I am reminded of the noise made by certain examples of Morris Minor pulling away in 1st gear (and probably equally so in reverse).......a noise as characteristic as the 'trumpet' exhaust note from the same vehicle. The Mike Leigh film 'Nuts in May' was recently shown again on TV and said noise was evident throughout. In days of old it was something to be mildly proud of to be able to quietly and smoothly double declutch into 1st gear at maybe 5 or 8mph on gearboxes of this ilk ....3 speed 100E Fords fell into this category....my father once slowly approached a junction and managed to select reverse instead of 1st and probably wasn't the first or last to do so bearing in mind that reverse was where 1st would have been on a 4 speeder.
|
>>I suppose it's difficult to make such things tough enough whilst keeping undue noise to a minimum, whilst keeping weight down, and at the same time keeping the box easy and light to use, a heap of compromises.
The biggie being can it be made cheaply enough.
The end thrust caused by helical gears does produce a small reduction in efficiency, but, in most cases, this is more than offset by the reduction in noise (for preliminary design purposes, I would assume a spur gear pair to be 99% efficient, and a helical pair 98%). As 659 alludes, if you take the tooth form of a straight cut spur gear, and make it helical, straight away, you get an increased contact ratio (the average number of teeth in mesh at any time), and hence, a smoother, quieter drive.
For example, the animation about half way down this page;
www.mech.uwa.edu.au/DANotes/gears/meshing/meshing....l
shows a spur gear pair with a contact ratio slightly over 1 - for a brief period of time in each cycle, there is simultaneous contact at more than point. For a given gear pair size, obtaining higher contact ratios by tweaking the design of the gear teeth usually results in weaker teeth.
|
Morris Minors didn't have syncromesh on 1st gear, like many others of that era, hence the whine.
Long-stroke engines meant good lugging power and many drivers in my memory (including my late father-in-law, most unsuitably, in my Hillman Imp) didn't bother to use the 'crash' first gear at all.
I remember one occasion, when I was about five, my great-aunt's heavily-loaded Austin 10 ground to a halt on a hill, crossing Dartmoor on the 'pretty' way to Plymouth. It took a while before my father realised she just didn't know it had another gear below the one she thought was first.
|
>>Morris Minors didn't have syncromesh on 1st gear,..., hence the whine.
The two aren't directly related.
>>Long-stroke engines meant good lugging power
Again, the two aren't directly related.
Edited by Number_Cruncher on 26/04/2009 at 13:23
|
|
|
|
I thought the whine on Moggie Minors and their ilk was due more to wear on the layshaft bearing.
|
The whine on Morris Minors was primarily due to the spur gear limitations discussed above. However, these gearboxes not only did not have synchromesh on first gear but had a sliding first gear and a reverse pinion which ran (when engaged) on first gear on the mainshaft.
Thus, there were two routes for serious tooth damage - crashing into first whilst moving would chip the leading edges of the mainshaft first gear and doing the same thing in reverse would further chip first gear and damage the (stepped) reverse pinion. I have rebuilt a few of these gearboxed in my time and have never seen a well used one without appreciable tooth damage. Although never quiet when in good condition, damaged teeth really made a noise - mercifully everything was out of mesh when the upper ratios were in use.
Rotten penny pinching design.
I learned to drive on a box like this, and although my cars have subsequently had a synchronised first gear for decades, I still sometimes wince when engaging first on the move.
659.
Somebody mentioned the end thrust generated by helical gears. My mention of M. Citroen above, deals with this. The Citroen trade mark is a grahic representation of two teeth from a double back-to-back helical gear which he used in some of his transmissions. Clever stuff - you get the smooth running of helical gears with zero end thrust as the forces become precisely equal and opposite.
Edited by 659FBE on 26/04/2009 at 13:53
|
|
Is thuis not simply down to the gear ratio? Some very old cars whine in first, because the final drive is indirect.
|
>>Rotten penny pinching design.
I disagree - I think they were OK, and I consider a lot of modern gearbox practice to be needlessly expensive and profligate. Although many do it frequently, the need to change into 1st on the move is rare, and for reverse, non existant.
Praising Citroen's gears!, is there no end to your profligacy?
|
Quote:...""Morris Minors didn't have syncromesh on 1st gear, like many others of that era, hence the whine.""
It's not the lack of synchromesh in itself that causes the sharp while of a gear, but the cut of the gear teeth. If they are straight-cut they make more noise than if helical cut.
The Mini-Metro had a loud whine, not unlike a Morris Minor, in first gear, but it had synchromesh on all gears.
Another thing was that old cars with rear-wheel drive and 4-speed gearboxes normally had a direct top gear, where the input and output shafts were locked together - so that there was no gear whine at all in top, ie 4th, gear. Modern cars mostly have all-indirect gearboxes.
Edited by Sofa Spud on 26/04/2009 at 14:17
|
With the benefit of a few decades' hindsight, I think it was a rotten design. You or I would hardly ever crunch such a transmission but it was clearly not "user proof" - hence my comment. I have never ever stripped a BMC transmission of that era to find undamaged teeth on first gear - and a pile of swarf in the bottom of the box.
To bring the matter up to date, first gear synchromesh is pretty well essential on a turbocharged diesel vehicle if you don't want to stall it at low speeds. Second gear just will not do - there is no boost pressure in second gear at low road speeds and insufficient torque is available to make progress. The no-speed safety trip fitted to electronically controlled diesels will then smartly cut the fuel.
Put a loaded trailer on the back and all becomes 10x worse.
659.
(Loves proper Citroen engineering - at a distance).
Edited by 659FBE on 26/04/2009 at 17:55
|
Slight topic drift here, but the BBC used to have a sound effect of car driving off that included a whining bottom gear. More importantly though it only suited some vehicles and sounded absurd with others.
Movie sound effects during car chases were also very often total rubbish in the fifities, and some film technicians are still very offhand or stupid about them. The thing that really got on my nerves was random tyre squealing noises, often when the car was driving in a straight line. By the same token a lot of actors swing wildly at the wheel when asked to act driving. And of course directors sometimes yield to the temptation to use speeded up film which always looks ridiculous.
Finally, cars that explode in a ball of flame after crashing, very unusual in real life, seem wall to wall in some movies. They annoy me as much as the lavish sub-machine gun fire that ruins so many crime movies.
|
|
Glad it's not just me Lud. I had thought I might be jiust a bit anal about these things. The one I simply can't bear is tyre squeals when the vehicle is clearly on a loose surface.
|
|
And to add to pet hates in motoring film.....trucks in the US that have no brakes and the inability to steer round an obstacle/person lying in the road.;)
|
>>tyre squeals when the vehicle is clearly on a loose surface.
I was amazed the first time I heard tyre sqeal at a grass autotest.
Believe me, it happens!
|
|
doesnt sound like a tarmac squeal though does it :-)
|
|
damm hb youve posted what i was just thinking
|
|
|
|
|
|
|
|
|
