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ME7 (Mechanical) (OP)
6 Dec 07 14:12
Does Helical Gears provide better motion quality vs Spur Gears? If so, how does one calculate the difference?
Helpful Member!(2)  digger200 (Automotive)
7 Dec 07 7:54
If they are the same grade etc then yes, as the total contact ratio of a helical set is greater than the spur, thus evening out errors.  However if you had a good ground spur set and a cut helical then no. It all depends on the design & accuracy
israelkk (Aerospace)
8 Dec 07 14:28
Do not ignore that with helical gears the bearings have to take linear loads added to the radial loads.
ME7 (Mechanical) (OP)
10 Dec 07 8:56
Thank you all for taking the time to assist me in understanding the differences.

israelkk,
Are you implying the losses that occur from the thrust load which has to be accounted for the lost energy in the effiency?

Thanks,
ME7
Helpful Member!  Javier72 (Industrial)
11 Dec 07 2:38
It is not so easy to see if an helical gear is going to have better or worst efficiency. Of course, you are going to lose energy by transmitting force to the bearings, but it is going to work smoothly. Normally, helical gears don't have so good efficiency as Spur gears, but it is not always like that. As it is said by digger200, it depends very much on the design and the accuracy of each component.

It is important to be very carefuly when manufacturing helical gears because little differences on angles can be very dangerous for the correct working of the engagement.
ME7 (Mechanical) (OP)
11 Dec 07 12:50
Thank you for sharing your insight. I greatly apprepiate the help.

Regards,
ME7
tbuelna (Aerospace)
22 Dec 07 6:16
ME7,

If you have high pitch line velocities, then a helical mesh is preferrable over a spur mesh.  Since the dynamic tooth loads are much lower.  You can also use a wider face width with helicals than you can with spurs, since they are less affected by torsional wind-up.  The practical L/D for spurs is about 1.0.  With helicals, the L/D can be closer to 2.0.

If you use a double helical mesh, then there is no net axial force produced by the mesh.

As for efficiency, a well designed helical (equalised sliding or recess action) is virtually as good as a spur mesh.
CPosner (Mechanical)
7 Jan 08 21:46
Someone correct me if I am wrong here, but helical gears are also quieter than spur gears.  So if operating noise is and issue, keep that in mind.  
Javier72 (Industrial)
8 Jan 08 2:38
CPosner, it is true that normally helical gears are less noisy than spur ones, but if they have similar correct design and manufacture. It could be so noisy an helical gear not very correctly designed or manufactured as a spur one with similar characteristics, and normally is easier to design or manufacture spur gears. When all the parameters are correct in designing and manufacturing, the helical gear is going to be quieter and strobger than a spur one.

plasgears (Mechanical)
8 Jan 08 11:18
Helical is essential for noise reduction. I worked on a superquiet multistage gear drive with helical composites, plastics, and extended pinion addendums. Curiously, unheat-treated steel pinions also contributed to low noise.

The key is use helical, extended addendum, composite materials, and materials with high internal damping such as aluminum, brass.
CPosner (Mechanical)
8 Jan 08 15:22
For those who aren't aware..
A key example of helical noise versus spur gear noise is in an automotive transmission.  Usually the reverse gear is a spur to you get that noisy whine as apposed to forward gears when you don't hear a gear meshing whine.
Helpful Member!  Matt51 (Mechanical)
21 Jan 08 9:05
Helical gears have a larger normal force, for a given torque, than spur gears. The helical Wn is the Wt/((cos (phin))*(cos(psi)), where Wn is normal force, Wt tangential force, phin is normal pressure angle, psi is helix angle. So the helical gear normal force, would be the same as the spur gear normal force, but divided by the cos of the helix angle. For a 20 helix angle, the helical gear would have a 6% greater normal force, and 6% greater frictional loss (not much difference).
As others said, helical gears are mandatory if noise is an issue. At very high pitch line velocities, helical gears are better in that they are less likely to trap oil in mesh.
Tooth deflection is an issue. Spur gear teeth come into contact all at once, and the best tooth modification for max torque is not the best modification for part torque. Helical gears, whose tooth contact starts as a point and gradually puts load across a tooth, can have a corner modification - or just an end easing, and still have unmodified profile that is optimum for all torque loads.
tbuelna (Aerospace)
27 Jan 08 4:46
A spur gear mesh can be made to operate quietly.  The perceived noise from a spur gear mesh is the gear vibrating due to sudden release of strain energy as each tooth comes out of mesh contact.  The helical tooth releases this strain energy less suddenly, due to its contact characteristics and higher effective contact ratio, so it produces less noise.  

To make a spur gear less noisy, you can decrease the gear tooth's pressure angle.  The gear tooth's bending strength will be reduced, but the contact ratio of the mesh will be greater.  So the release of strain energy will be less pronounced as each tooth comes out of mesh contact.

As an example, high capacity gears are designed with pressure angles as high as 25 deg., if noise is no concern.  On the other hand, where noise is a concern, but not so much  bending strength, a pressure angle of 14 deg. or less may be used.
Gerrit55 (Mechanical)
13 Feb 08 17:24
Hi, have you ever tried two helical gears on one shaft, one is lefthand and the other one righthand both 15 degrees. this might give an amazingly quiet running reducer. This in accordance with Geartechnology sep/oct 2007 pages 68 through 74.

Matt51 (Mechanical)
14 Feb 08 10:34
Gerrit55,

I assisted in design of a gearbox that had two helical gears on a shaft, both same hand so the axial loads offset each other. Helix angle of one of the gears was 15 degrees, the other was larger. Both had face contact ratio 1.0. This helical gearbox replaced a spur gearbox, and there was an order of magnitude reduction in vibration levels. I am convinced of the superiority of helical gears, in applications where noise or vibration are an issue.  

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