FEATURE: Rally transmissions explained
- 23rd August 2011, 9:00am
Gear set design has come a long way in the last two decades. Most of this is due to improvements in CAD technology and gear design techniques. Up-to the minute design trends are essential for gear manufacturers breaking new ground and exceeding limitations.
High-end manufacturers use billet material to form gears. Truth be known, a good deal of manufacturers use very similar material and in fact there are a limited number of steel mills globally, producing the high-alloy material favoured by gear manufacturers today. Traditionally, high-volume manufacturers use forgings which results in a cheaper product that can be mass produced, but one which proves unfavourable when manufacturing driveline products for extreme duty.
Most gear manufactures “cut” gears on CNC gear cutters, this machining or “cutting” is done using a tool which may often need to be designed specifically to cut the particular “shape” of the tooth. Tooth design is critical to power holding capability and the design itself requires the following factors be taken into account: shaft distances, ratio, tooth count and pressure angles, amongst other things. Poorer quality gears are often manufactured using off-the-shelf tools to save on costs, whilst compromising tooth strength.
After final machining, the shafts are sent to heat treatment for hardening. This, again, is now computer controlled in most cases and is critical to gear strength. However, after heat treatment there is often measurable distortion. This is an area that is relatively new to gear manufacturers. Gear distortion under heat treatment is an unavoidable physical change that can be exacerbated by the size and length of the tooth and gear itself. Manufacturers that cannot or do not measure distortion are merely relying on the end product being somewhat similar to the cad design it was intended to be like. In this situation there can be unexplained inconsistencies in strength and noise, as well as wear.
Specialized Computerised Measuring Machines (CMM) are now available (although costly) which measure tooth and gear imperfection and through constant analysis, it is now possible to imperfectly cut a gear that is known to behave in a certain way to heat treatment, which then results in a gear closer to perfection after it undergoes the physical changes of distortion. This process takes time, like three or four batches of product to perfect. The result is a quieter running gear with better tooth contact and superior strength.
How can distortion be avoided? Well it can’t, but what we can do is “gear-grind” the gear. This is a different process compared to cutting with a tool or hob.
Gear grinding is amazing to watch. The machine literally forms the gear by grinding it to perfection after heat treatment. Distortion is almost zero and the product could almost be deemed perfect in every respect. Grinding also allows design changes, like the manufacture of gears and shafts in one-piece in situations which would normally require components be “cut” in different pieces, and then EB (Electron Beam) welded together. This is most often the case with synchromesh gears where gear cutting of the gear itself can interfere with the synchro gear adjacent to the gear.
These machines are not cheap, with some gear grinders costing a few million dollars. (pictures attached show Albins CMM gear inspection machine and Samputensili Gear Grinding Machine located at Albins, Victoria)
The information above relates to gear strength. What is probably the next most important factor when choosing a gearbox is maintenance. This most often is related to dog wear when we are talking about rally car transmissions.
Dog gear engagement is facilitated by numerous large teeth (dogs) that mate into matching openings machined into the opposite surface of the driven gear. Unlike the synchro engagement, the two rotating gears are operating at different speeds (unless the revs have been matched) and there is no synchronising mechanism to assist in bringing them up to a synchronised (equal) speed.
The number of dogs (teeth) and the size of the openings determine the window of opportunity that the dogs have to engage on the shift event. It is for this reason that often we find a smaller number of dog teeth offers a better (easier) shift quality. The downside to this easier engagement is an increased noise and abruptness on the shift. It is the profile and design of these dogs that are unique to each manufacturer. Finding a balance between performance and longevity is the key.
If the dogs do not line up to facilitate a gear engagement, the faces of each opposing surface (dogs) will clash and over time can wear. Wear will depend on the speed of the dogs in relation to each other and the force applied.
A unique feature to all Albins dogs are a CAD designed “pent-roof” pentagon shaped surface used to deflect the dogs apart on a miss-shift. This drastically enhances dog gear life. All Albins dog gears are verified using a robotic CMM (co-ordinate measuring machine) to assure perfection with tolerances better than .015mm on each and every part. Any misalignment and poorly machined parts will drastically effect performance and wear characteristics on dog engagement systems. Tolerances on synchro engagement systems are much more forgiving due to the slipping of the brass cones.
Ideal gear selection to minimise clashing and wear of the dog rings is achieved by a momentary break in the engine's driving load removed until the shift is completed. This is achieved by a quick throttle blip or clutch depression. (The opposite is true of a synchromesh gearbox as used in passenger cars, where slow movement helps to allow the synchros to match shaft speeds). When timed properly with practice, the movements are very quick, measured in milliseconds.
Remember, there will be no dog wear when the dogs are fully engaged (car is in-gear). The damage can only take place when initiating contact during a shift (miss-shifting), therefore this event must be made as short as possible. If a driver moves the gear lever slowly, or if the linkage is poorly secured, dog wear will occur in various degrees. It is probably worth mentioning here that dog wear is inevitable to some degree, but shift “style” amongst other things will have a bearing on the amount of wear experienced.
Swift gear shifting is desirable when using a dog-box and the clutch may or may-not be used when shifting gears.
Finally, anti-fatigue shot peening processes are unique to each manufacturer and in some cases are not undertaken by manufacturers for some reason that is unexplainable, except to say that it reduces the end-cost. Post-manufa cture treatments such as isotropic polishing and shot peening are sometimes what separates quality parts from parts that just do not last. These processes increase strength in the alloys by aligning particles (at a microscopic level) minimising chances for hairline fractures and cracks.
The shot peening treatment gives gears a brownish look... much like factory gears. It may not look as attractive as a beautiful silver gear, but it certainly extends service life significantly.
Credit to www.albins.com.au for supplying images and technical information. Albins may be contacted via email@example.com and 035 335 8022.
- Article provided by Albins.
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