THERE are few innovations that have made such a major impact on the engineering of a modern car than the helical cut gear.
The helical cut gear has allowed people to enjoy cars that are much quieter and more refined compared with the hunks that our grandfathers drove.
However, most of us don’t know what helical-cut gears are and why they are so important, so here is where I come in.
I’m just kidding. You guys are classic car fans, so you think I’m completely crazy and you are flicking through your dog-eared references to point out to me how straight-cut gears are more efficient at power transfer.
And, just so that I know, you would point out the fact that no proper racing car has even been caught using helical-cut gears.
Of course, you are right but not really for the reasons that you think you are right.
Straight-cut gears are slightly more efficient at transferring forces because they do not generate much axial load, but as most engineering books will tell you, both types of gears have between 97 and 99 per cent efficiency.
Some may say that straight-cut gears are stronger and that is why it is used in racing cars, but actually, the gears are stronger than road car gears not because of the way they are cut but due to their manufacturing process.
Helical cut gears are stronger because they have far more surface areas and due to the helical design, more of them are in contact at the same time, spreading the load.
In a straight-cut gearbox, the number of teeth in contact varies between 1.2 and 1.9.
If only one tooth is in contact at a time, the way the teeth slams into each other would make the gearbox last a short time.
Making more than two teeth mesh at the same time on a straight-cut gears would wear out the teeth and also make them less durable.
Straight-cut gear are fast acting and feel like gears are slamming into each other because they do not suffer from the gentler meshing of gears that relies on the axial losses to make them mesh smoother.
In a helical cut gear set, the number of teeth that are in contact at any one time is between two and four.
The angle of the helix determines the number of gears that are in contact at any one time and the rate of their contact.
The more acute the angle, the more teeth would be in contact and the gentler their engagement.
If you cut across helical gear, you can count how many teeth would be cut in that straight line and that is the number of teeth that would be in mesh.
So, if helical-cut gearboxes are more durable and stronger, why don’t race cars use them? I hear you ask with a tinge of gloating, thinking that you may have gotten me this time.
The answer is weight, and not gear efficiency or strength.
The axial load generated by the helical-cut gears means that it needs a robust casing, otherwise the shafts would punch through them.
A helical-cut gearbox of the same configuration would require a heavier casing and no one needs more weight in a racing car.
The axial load generated by the helical transfer of force would also require beefier bearings that are more complicated because they need to handle sideways forces. That’s more weight that race cars don’t need.
Anyway, the whine of the straight-cut gearbox is music to a race car driver’s ears, so it’s not fair to deprive them of their on-job entertainment.
As child of the 1970s, I remember straight-cut gears in some of the older buses then and how they whined about everything that was wrong in the world. After a few hours, that gets old.
In the old days, gears were cut then tempered because cutting tempered steel was a lot harder. The downside is that the gears do not retain their shape 100 per cent and the slight change causes imperfect meshing and this also results in whining.
Nowadays, many companies cut their gears almost to the final shape, then have it tempered and do the final machining after that to ensure more accurate meshing that significantly reduces noise and vibration.
Is there no way to counter the axial load of helical gears?
Yes, there is, you can use herringbone gears, which are a pair of helical cut face on the same gear but they are more complicated to make. They are much wider and the additional weight and complication means they are just not worth it.
All hail the helical cut gear for long will it reign.