Helical Gears - Physics of why smoother and quieter 4

[ME7]

I am trying to understand in further detail why helical gears are smoother and quieter over spur gears. What is the physics that differentiates "smoother" and "quieter"? Curious because many printer designs use helical gears over spur gears supposedly for better motion quality.

Please help.

Thanks!

 

[CoryPad]

Helical gears offer a refinement over spur gears. The leading edges of the teeth are not parallel to the axis of rotation, but are set at an angle. Since the gear is curved, this angling causes the tooth shape to be a segment of a helix. The angled teeth engage more gradually than do spur gear teeth. This causes helical gears to run more smoothly and quietly than spur gears.

 

[MikeHalloran]

Or, think of it this way:

In a spur gear pair, there's a click or shock as the load transfers from one tooth pair to the next. The shock may or may not be smaller as the lash decreases or the number of teeth increase.

Imagine that you've split a pair of spur gears on their major planes, indexed them half a tooth, and pinned them back together. You've doubled the number of shocks per revolution, and maybe reduced their size.

Now keep doing it, splitting the gears again and again, indexing them slightly and rejoining, say in a spiral pattern. At some point you've got enough gear pairs that the teeth overlap, i.e. gears at one end of the stack are engaging just as gears at the other end are disengaging, so there are (nearly) always two gear pairs carrying the load. Now make the number of gear pairs infinite and reduce the width of each pair to almost zero. You've got an infinity of slightly staggered gear pairs, at least two of which are always carrying the load: helical gears.

The tooth action at any given cross section is no different from a spur gear, because the profiles are the same, but the teamwork of an infinity of infinitesimal gear pairs keeps the load always 'supported', greatly reducing the noise generated.

Of course it doesn't work if the width and helix angle are such that the gears are 'short', i.e. you can see something other than a perfect cylinder in axial projection, or if the gears are excessively compliant in torsion, but you get the idea.





Mike Halloran
Pembroke Pines, FL, USA

 

[ExRanger]

One way to think of it is that with typical spur gears, at some instants in time you will have 1 pair of teeth making contact and at other instants in time you will have 2 pairs of teeth making contact. There is typically a rather abrupt change from 2 teeth to 1 and vice versa. Imagine you have 2 pairs of teeth carrying the load and suddenly one pair goes out of contact leaving you with much less stiffness in the contacting teeth (2 pairs of teeth being stiffer than one). In that moment the deflections at the tooth mesh increase and the gears have momentary acceleration.

With helical gears, you usually have at least 2 pairs of teeth in contact at one time, and the change from 2 to 3 (or more) pairs in contact is smoother and more gradual. A pair of teeth doesn't suddenly drop out of or come into contact, it peels away along the helix or gradually blends in. This leads to generally smoother meshing action.

Having said that there are ways to make spur gears engage very smoothly too.

 

[ME7]

Makes great sense.

Thank you for your time and help.

ME7

 

[mfgenggear]

nice explanation I gave you all stars

 

[tbuelna]

ME7,

As the other stars have noted, helical meshes are quieter than spur meshes primarily due to their higher "face contact ratios". Gear meshes generate acoustic disturbances (ie. noise) due to the structural vibration response of the gear body caused by the sudden release of strain energy as each loaded gear tooth rotates out of mesh contact. A helical gear tooth mesh has more teeth sharing the load across the pitch line (ie. face contact ratio) at any given instant than an equivalent spur gear mesh. So the amount of strain energy released as a result of a single tooth coming out of mesh contact is also lessened. And naturally, the vibratory response of the gear and the perceived noise that it produces are much lower, as these things tend to be logarithmic in nature.

I would tend to disagree with the posters that state a helical gear tooth has a more benign contact motion than a spur gear. Both types of gears are based on the same involute geometry and have the same profiles in plane. So their approach and recess actions are similar. The only difference being that helicals are less sensitive to torsional deflections along a wide face than spurs.

Dynamic tooth loads are normally much greater than static loads, due to tooth index errors. What this means is that at any given instant, some teeth may be much more highly loaded than the adjacent teeth as they pass through the mesh point. So with regards to gear noise, index, profile, and pitch line errors, as well as housing and bearing stiffness, are very significant factors.

Regards,
Terry

 

[plasgears]

Note the benefits of helical coupled with interaction of soft pinions and composite/plastic gears. We had a gearbox that ran so quiet that you could not hear the gear noise. Another trick is modifying pinions with extended addendum.

 

[Spurs]

Note to all:

Your explanations of why helical gears are quieter are very theoretical in nature.

In actual practice, I would tend to disagree with such a large and all encompassing statement.

The fact is that most gear noise is generated due to inacurate profiles whether spur or helical gears are used. However, helical gears are inherently more difficult to hold precision compared to spur gears. Hence any percieved improvement in noise is quickly lost when an apples to apples comparison is made.