Why don't motorcycle transmissions use helical gears?

[marshall]

I know a couple big cruisers have been made with helical-gear transmissions, or aftermarket companies have made conversion kits. But why do most car transmissions use helicals, while motorcycles use straight-cut? Here's my understanding:

Straight-cut:
-Easier to manufacture
-No axial forces to deal with
-Less demanding of lubricant (?)

Helical:
-Quieter
-Smoother
-Less localized tooth stress
(all of the above due to increased contact ratio)

Do helical gears require more accurate axis alignment and center spacing?

Are helical gears less capable of large reduction ratios due to tooth geometry (motorcycles must step down speed of rotation about twice as much as cars)?

I thought I read somewhere that helical gears were generally better for high speed applications, which would make them better for motorcycle applications. Maybe I misread and it's the opposite...

Just curious.

Marshall

[Guy_b_g]

This is what I found on Wikpedia:

Helical gears operate more smoothly than do spur gears. 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. It may span the entire width of the tooth for a time. Finally, it recedes until the teeth break contact at a single point on the opposite side of the wheel. Thus force is taken up and released gradually. With spur gears, the situation is quite different. When a pair of teeth meet, they immediately make line contact across their entire width. This causes impact stress and noise. Spur gears make a characteristic whine at high speeds and can not take as much torque as helical gears because their teeth are receiving impact blows. Whereas spur gears are used for low speed applications and those situations where noise control is not a problem, the use of helical gears is indicated when the application involves high speeds, large power transmission, or where noise abatement is important. The speed is considered to be high when the pitch line velocity (that is, the circumferential velocity) exceeds 5000 ft/min.[5] A disadvantage of helical gears is a resultant thrust along the axis of the gear, which needs to be accommodated by appropriate thrust bearings, and a greater degree of sliding friction between the meshing teeth, often addressed with specific additives in the lubricant

Other sites mentioned the need for stronger gear cases (with helical gears) because of the high end plate loading which require heavier cases to avoid case/bearing failure.

The helical gears are also supposedly slightly less efficient in transmitting power due to higher friction losses, but I think this is pretty minimal..the reason being that I used to have a Moto Guzzi Centauro which had helical cut gears. Moto Guzzi also made the Daytona, which had straight cut gears and was a sport model. Other than that and a higher gear ratio (plus a hotter cam in the European Daytonas) there was no difference in the power train. On dyno runs of the Daytona and the Centauro with the US spec cams the difference in horsepower was negligable at the rear wheel. The Daytona shifted a bit better/faster than the Centauro, the Centauro was quieter (not much though because Moto Guzzi left the straight cut gears on the cam drive and they whined like crazy...I loved that sound but...go figure)

Hope this helps...

[oseaghdha]

Helical gearing is much more expensive to engineer and manufacture, with tighter tolerences and more elaborate machining and inspection methods required.

[TADracer]

And then there is the cost. A helical gearset probably has a much higher initial machining cost due to all the compound angles then add that to the extra design, thrust bearings, heavier cases and on and on. Might be a cost vs advantage thing and if that is the case, you know which way the manufacturers will go.

[Ivan]

The part about high speed being over 5000 ft/min was interesting. 5280 ft/min is 60 mph, and with the small circumference of transmission gears, that would be lotsa r's. I highly doubt our XS11 tranny gears spin anywhere close to that.

One thing that wasn't mentioned, is that spur gears can be forged and then the teeth finished machined. Forging is much stronger than billet and is much quicker and cheaper than a fully machined piece from billet.

[BigDick]

When I worked machining gears, there was not much bigger difficulty in setting up to cut helical gears than it was to cut spur gears. I would imagine that the thrust bearing problem and having to have stronger cases were the prohibitive factors here.

[WMarshy]

suppose to learn something new every day right?

Hi guys, interesting topic we have here. For what its worth, I did a refresher/dig in my machine design book and found a few things that would steer Yamaha engineers towards spur gear over helical gears. I will attempt to elaborate some on what Ivan mentioned incase not all of us have been delighted to peruse the light pages of a machine design book. (I don’t claim to be an expert in this area, just enough knowledge to be slightly dangerous)
Ivan pretty much hit the nail in the head; it’s all centered on cost and how “the product” will be used/abused. The American Gear Manufactures Association (AGMA) dictates that gears meet a quality index which is determined by method of manufacture. The quality index recommends a maximum pitch line velocity for a gear. Therefore, a faster turning gear needs to be more precisely manufactured. The AGMA recommends automotive transmissions meet a quality index of 10-11, and a pitch line velocity of 2000-4000 fpm. Shaving or grinding is capable of achieving this quality index.
Im not that familiar with the size of our drive gear in out transmissions but did a little calculation to check it out… A 1.5 inch drive gear rotation at 8000 rpm results in a pitch line velocity of approximately 3100 fpm.
Velocity = (pi * gear dia * rpm)/12 Result: feet/min.

The book says spur gears are seldom used above 10,000 fpm due to excessive noise and vibration.

[planedick]

It's a Gleason

That's the name of one of the machines used in the manufacture of the helical gears. I worked in a facility where they were used and they are large, expensive and slow to make a gears. I was told that this place was one of only two west of the Mississippi, but then that was in '66. I agree, cost is probably the main factor.

[BigDick]

That's the name of one of the machines used in the manufacture of the helical gears. I worked in a facility where they were used and they are large, expensive and slow to make a gears. I was told that this place was one of only two west of the Mississippi, but then that was in '66. I agree, cost is probably the main factor.

Our machines were mostly Koepher, we did have one Pfauter. Our largest diameter gears were about ~5" diameter. All of ours were used in house.

[fredintoon]

Helicals cost more. They can get away with using straight-cuts on bikes because a bike is typically far more liable than a car is to be scrapped by neglect or collision damage before it actually wears out and it also does far less mileage than a car does anyway.