7 replies to this topic

[marksmini]

Hi all,

im curently teaching Transmission to a class and ive been hit with a question i cant answer fully,
so thought i would ask here to see if anyone else knows.

what are stright cut gears loud?
please dont tell me about them being a spur gears and helical gears i know the differance and how they work, and also the pro's and cons of each of them but unsure why the noise. i think its to do with how they mesh together? but cant be sure on this question,

can anyone help or advise why there so noisey?

Mark

[dklawson]

I don't think your question can be answered once you said not to include statements about the differences between spur and helical gears.

Moving away from those keywords (as much as possible), lets describe the difference that spur gears have limited points of contact that hammer into each other as the load point is transferred from one mating pair of teeth to the next. By comparison, the helical gears move more slowly into contact and operate with a greater amount of contact area than spur gears. In short, the helicals slide together while maintaining multiple points of contact while the spur gear teeth tap together and pull apart as pairs of teeth move in and out of contact.

[Dan]

Ultimately it's largely because they are made down to a price. They aren't in constant mesh like helical gears are so there is a time, however brief, in between the load being taken by teeth that the driving gear will accelerate away from the driven gear while the driven gear is slowing. The greater the speed differential between the gears that develops during this period the harder the driving gear will strike the driven gear when they re-engage and the louder they will be. It is possible to reduce the time that the gears are out of mesh to a minimum by creating elliptical section teeth (take a look at the gears that come in Lego sets because they have a few different tooth profiles, see how tightly they fit together and how little lash there is in the gears having elliptical teeth compared to the flat section type). Elliptical teeth can't really be cut easily by a machine though and must be cast and individually matched, so nobody could make a gearbox like this. They are loud because they are constantly hammering against each other.

[pantera2075]

Hi all,

im curently teaching Transmission to a class and ive been hit with a question i cant answer fully,
so thought i would ask here to see if anyone else knows.

what are stright cut gears loud?
please dont tell me about them being a spur gears and helical gears i know the differance and how they work, and also the pro's and cons of each of them but unsure why the noise. i think its to do with how they mesh together? but cant be sure on this question,

can anyone help or advise why there so noisey?

Mark

With Helical gears there is always some portion of a gear in mesh, whereas with straight cut they are continuously coming out of, and going back into mesh. The noise is the action of the gears contacting and rolling over each other when going into mesh. Helicals are less efficient due to the losses caused by side forces induced by the angle of the gears.Note that individual gears are not "triangular", their profile is more akin to a sine wave - but chopped off.

It'd help if you got some diagrams of how gears intermesh.

[Guess-Works.com]

And there's something else you have also forgotten...

Not only is the noise attributed to the gear profile, but more importantly to the oil which is getting squished between them... With Helical gears at any one time the whole gear is not engaged, but a small part of a number of gears... This allows the oil to escape...

If you look at an A series gear verses an A+ gear... the A series are blocky, where as the A+ are pointed and have a grove... and a different cut angle.. this is all done to allow the oil to escape thus reducing noise... and hence why an A series gearbox still whines and an A+ does not...

SC gears basically squeeze the oil out sideways between the gears, and is quite possible that it's supersonic when it exits...

Modern designed SC gears take this into account and are appreciably quieter than some older designs...

The Lash mentioned above contributes to the noise when a gearbox is not under load, ie the rattle you sometimes hear...

Edited by GuessWorks.co.uk, 30 April 2008 - 01:08 PM.

[Aus]

As above but I'd add to what guessworks said about oil in so far as the actual gears in the box are also fairly well immersed in oil that help damp sounds. The SC transfer gears are the real noisy ones as they dont run in oil, relying instead on oil brought and sprayed up from the lower gear that is in the oil in the box.

Basically well made gears are much quieter than cheap ones as they mesh with much less play and the contact surfaces are a greater area.

[marksmini]

well. i think thats blown that question out of the water.

thank you everyone, i fully understand now. well explained !

[Retro_10s]

Worthy of FAQ me-thinks!