What specifically, is the design principle behind the Helical shape of gears in a synchromesh transmission?

Are the Helical gear designed, in any known vehicle, to disengage? Is that why the gears are helical? To make the engage/disengage smoother?

As far as I know, the helicals on any synchomesh don't ever disengage. Their linkage with the dog-clutch/blocking ring/mesh cone -- Basically the dog-clutch teeth is the only part that disengages. Once the dog-teeth disengage, that will allow the entire helical to free-wheel on the mainshaft. But the helical teeth NEVER come apart.

2 Answers 2


Not only do helical gears run more smoothly and quieter than straight cut gears (as said in Nick C's answer), they also provide more surface area than do straight cut gears at the mesh point. This larger surface area means less chance of gear failure due to stress risers. In a straight cut gear, the load is transferred from tooth to tooth directly. In helical cut gears, the load is divided across different teeth which also makes it a stronger mesh. No single tooth is providing support for all of the load.

You can see what I'm talking about born out in this side by side comparison:

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Even though the gears are the same width, the tooth length is longer for the helical cut gears. This provides more strength and better load distribution.

  • Since you brought in contact patch, you could maybe add that the contact patch (decided by the helix angle) is shared by a few teeth and is sliding(wrt a tooth). This in turns produce axial thrust of gear shafts and effects efficiency (Output shaft power / Input shaft power)* 100
    – chilljeet
    Commented Jun 25, 2015 at 6:09
  • While the effect on efficiency is probably negligible, the axial thrust is probably the only reason limiting their use in high torque applications. This is probably why spur gears are used for racing applications, not because they;re stronger. I might be wrong and missing other points
    – chilljeet
    Commented Jun 25, 2015 at 6:18
  • 2
    In citroëns I think mirrored helical gears (chevron shaped) were used, giving the benefit of helicals without giving the axial forces. At least that's what their logo is based on.
    – Allman
    Commented Jun 25, 2015 at 6:47
  • @Allman Did not know that (about the logo)
    – chilljeet
    Commented Jun 25, 2015 at 7:09

It's generally because helical gears run more smoothly, and quieter, than straight-cut gears.

Quoting wikipedia (https://en.wikipedia.org/?title=Gear#Helical):

With parallel helical gears, each pair of teeth first make contact at a single point at one side of the gear wheel; a moving curve of contact then grows gradually across the tooth face to a maximum then recedes until the teeth break contact at a single point on the opposite side. In skew gears, teeth suddenly meet at a line contact across their entire width causing stress and noise.

You'll notice this if you ever hear a car with a straight-cut 'box (such as many racing Minis) - they make a very distinctive whine...

  • Yeah, that's good. Can you say with a large degree certainty that that is the MAIN reason? Is it known that helicals disengage? Commented Jun 24, 2015 at 14:11
  • I've never seen any automotive application that involves gears of either type disengaging from each other, AFAIK all gearboxes use dog clutches to engage gear to shaft - but there might be some obscure one I've not come across!
    – Nick C
    Commented Jun 24, 2015 at 14:13
  • Right, so with large equipment where synchomesh is not appropriate, you have to double-de-clutch, and engage/disengage is accomplished with the dog-clutch. But you never see non-dog gear teeth (straight or helical) actually unmeshing. Commented Jun 24, 2015 at 14:21
  • @AndyzSmith - your next question could be , why use spur gears in racing applications?
    – chilljeet
    Commented Jun 24, 2015 at 14:52
  • 1
    The reverse gear on otherwise-helical manual transmissions is generally straight-cut, is it not? That may be a more familiar source of the typical whine.
    – jscs
    Commented Jun 24, 2015 at 19:09

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