Generally used only in motorsport applications, straight-cut gears are an interesting alternative to a conventional gearbox setup. Known for their distinctive scream and inherent lack of usability, this is a transmission setup that most of us will never come into contact with.
What are they?
The name explains it all really, the actual teeth of the gears point straight out from the centre point of the gear instead of forming a helical shape like standard gears in almost every other road car on the planet. Instead of the teeth curling nicely in a spiral format around the centre axis of the gear, they protrude outwards; more like the sprocket on a motorbike.
How do they work?
The main advantage of using straight-cut gears is that they produce no axial load. This ‘thrust force’ is generated by the sliding contact between the teeth of helical gears. This lateral force is applied to the input shaft of the gearbox, which in front-wheel drive configurations will then convert through to the driveshafts. This greatly restricts the amount of torque that can be applied through the gears before failures on other components occur.
So straight-cut gears effectively allow much larger powertrains to be placed in a vehicle without risking the output shafts and other bearings tearing themselves apart, producing a larger safety factor within the transmission itself.
The adverse axial load is depicted here in red, a force absent in straight…
Are there any other advantages and disadvantages?
A straight-cut transmission is inherently more efficient than a helical gearing system. The axial load produced by helical gears only detracts from the output energy from the transmission along with an increase in friction and therefore losses of energy due to heat occur.
Straight-cut gears are also much easier to assemble and produce less-catastrophic failures when they do go wrong due to their simple structure. Heavy-duty transmission casings and shafts have to be used with helical gears for the transmission to cope with the additional axial load, therefore straight-cut gears save heaps of weight which is extremely important in a motorsport setup.
Classic Racing Minis are often converted to straight-cut gearing
Drawbacks come in the shape of their convenience and operation. They naturally create a hell of a lot of noise; helical gears mesh together in small segments at a time while straight-cut teeth are in contact with each other fully and for a longer period of time which makes for a very distinctive whine. The operation of the gears is also fairly brutal when straight-cut; the gears appear to ‘slam’ into place after each change and can make pulling away smoothly or getting a decent launch difficult.
Technically, on a tooth versus tooth comparison, a helical gear can carry a larger load seeing as it is diagonally positioned on the gear (assuming tooth size is equivalent). This means that it distributes the forces being applied on it much more efficiently compared to a weaker, vertical tooth.
Steve McQueen looking cool as ever beside his Sebring Sprite which wielded…
Despite virtually all cars these days using standard helical gearing systems, there are still applications within motorsport that use straight-cut gearing. Back in the prime of sports car racing (1950s and 60s), Stirling Moss and Steve McQueen drove Austin Healey ‘Sebring’ Sprites to 3rd and 9th place respectfully at the 4 Hours of Sebring, a race specifically for cars under 1000cc. These little pocket-rockets sported lovely little four-speed straight-cut gearboxes and one such car was restored faithfully by CT favourite Fuzz Townsend on the last series of Car S.O.S.
Many motorsports enthusiasts these days will convert their cars to straight-cut gears for their efficiency when driven hard, as you can see (or more like hear) below through this converted BMW E36 320i racecar.
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I wouldn’t rush onto eBay to look for a straight-cut gearing conversion for your daily driver, but it’s certainly something to think about if you have a spare trackday weapon!
Why are gearboxes are needed? A gearbox is a way to increase or decrease torque, motor speed, and direction. What are the differences between gearbox types? Well, determining which kind of gear motor is most suitable for you will depend on the application you are doing. Every kind of gearbox has its own advantages and disadvantages.
Looking to install or upgrade your gear motor? Take a look at our Bonfiglioli gear motor selection today. Keep reading below for descriptions of the most common gear motors and their pros and cons.
Planetary Gears:
This gear design mimics the planetary rotation, hence the name. Planetary gears consist of 1-6 outer spur gears that rotate around a central gear, which is also referred to as the sun gear, and the outer gear, also known as the planet carrier.
This type of gear is compact, highly efficient, emits little noise, and provides excellent torque transmission.
Pros':
Huge torque capability
Good load distribution
Handles extremely large reductions
Small size
High efficiency
Ease of maintenance
Cons':
Generally more complex
High stress on the output shaft bearings
Thermal limitations on smaller sizes
Worm Gears
Worm gears are used in applications where large gear reductions, torque, and braking are necessary. Some of the more favourable features of worm drives are that they achieve high ratios in a single reduction, ability to handle shock loads and low noise;
One advantage that makes worm drives desirable over other gearboxes is that they have relatively few moving parts in high reduction ratios, simply two shafts – input and output – supported by four bearings.
Pros':
Ease of maintenance
Low noise properties
Handles large reductions
Large shock load capability
High thermal capacity
Cons':
Moderate Efficiency
Large package in design
Relatively expensive to manufacture
Spur Gears
Spur gears are the most common type of gear. These are used in the simplest of gearbox designs, making them the cheapest and most common to manufacture. Spur gears are straight cut gears that have teeth that is meshed with other spur gears to form the gearbox. This design allows the gears to decrease speed and increase torque.
While spur gears are inexpensive and common, they aren't always the best solution, especially in cases where high torque or speed are needed. Several spur gears are needed to make large reductions. They can be very loud while in operation, and if too much stress or force is put on the gears they can easily break.
Pros':
Inexpensive to manufacture
Easy to operate
Efficient and reliable at low speeds
Cons':
Require several gears for large reductions
Loud at high rotational speeds
Not able to handle large amounts of torque
Helical Gears:
Helical gears are similar to spur gears in that they are inexpensive to manufacture but have a few differences that give them an advantage. The teeth in helical gears are cut at an ‘helix’ angle to the leading edge of the gear so the gears to mesh together over the length of the helical cut. This allows for a smoother transfer of torque. Helical gears also makes less noise than spur gears do.
Pros':
Inexpensive to manufacture
Easy to operate
Good torque capacity
Less noise while operating
Cons':
Requires several gears for large reductions
Higher maintenance costs
Looking to replace or upgrade your gears? Take a look at our gear selection today or call Gordon Russell at in BC at 604 940.1627 or in Alberta at 403 340.8856 today!