Reason to avoid tip contact in gears

[k.udhay]

Hi,

When I was studying some gear drawings, I found the tip relief given for all the gears. I find the reason that it avoids the contact between tip of driving flank and root side of driven flank. What is the intention of avoiding this condition? What if I don't give a tip relief?
Thanks.

 

[maajdl]

A tip contact can make much more wear and even damage.
Pressure is of course high is such contacts.

 

[k.udhay]

//A tip contact can make much more wear//

What is the reason for this, Maajdl?

 

[maajdl]

stress concentration

 

[Baluncore]

Spur gears must always operate with a slight clearance. This does not significantly effect the rolling contact of ideal involute profiles. However, clearance has a side effect in that, as load is passed from one tooth to the next, it is possible for the incoming tooth tip to impact the mating dedendum surface. That will produce a significant meshing noise and stress the tooth. A progressive tip relief by a couple of thousandths of an inch, replaces the impact with a smooth engagement that is quieter and less damaging.

Tip relief is preferable to dedendum relief since it does not weaken the cantilever of the tooth.
Similarly, crowning relief can be used to reduce noise due to axial misalignment.

 

[k.udhay]

Thanks for a very clear explanation, Baluncore. Is it true that tip relief introduces / aggravates transmission error in return?

 

[Baluncore]

As the torque is transferred from one tooth to the next there is a period during which both teeth are in contact. Tip relief is only applied to the part of the tooth face where the contact is being made and the load is still being carried on the unrelieved portion of the previous tooth.

Transmission accuracy is unaffected so long as the tip relief is outside the region with single tooth contact.

 

[Baluncore]

It is the usual practice not to make the hob straight sided throughout the whole depth of tooth corresponding to the involute profile, but to introduce rounding near the root of the hob to give tip relief. This allows for deflection of the teeth of a gear under working load without causing the tips to strike hard at the point of entry into contact.

Whilst tip relief is essential for heavily loaded spur gears it can be of lesser amount in the case of helical gears, where end relief is more important since each tooth enters the contact zone at the leading end. Such (crowning) relief can conveniently be carried out by shaving, preferably on the pinion only. (See pages 79 and 80).

METHODS OF CROWNING
A feature which it has been found desirable to introduce into the shaving process is the ability to carry out crowning or end relief so as to avoid hard contact at the extreme ends of the teeth. This is done by causing the cutter to approach the work more closely at the points where it is desired to remove additional material. Slight crowning may take place without providing any special means, since when the cutter runs out at the ends of the teeth, the area of the contact is reduced, thus localizing the pressure which has been applied.
…
The amount of (crowning) relief needed in all practicable cases is very small, about 0.001 in. (0.025 mm.) to 0.002 in. (0.05 mm.) for wide gears and 0.0005 in. (0.0125 mm.) for narrow gears.

Tip relief and crowning are both introduced to reduce the percussion of teeth as they contact.
Tip relief is most important on spur gears operating under load.
Crowning is important on helical gears, or spur gears with minor deviations of shaft alignment.

 

[k.udhay]

Thanks a lot, Baluncore! As usual, the information you have given are of top-notch quality!