Gearbox Design
Discussion
Hello everyone!
This is a new project I have been given on my Mech Eng degree...I have to design a gearbox for a microlight...so if anyone has any suggestions or any help for me it would be great!Thanks!
'A speed reduction ratio of approximately 2.38 was originally specified for a microlight powerpack, however a new gearbox must be designed to accomodate a range of low speed high efficiency propellers. It is felt that the higher gear ratio now required may dictate the need to use an epicyclic gear set.
Complete a detail design of the gear set required for the application.
An outline scheme of the gear case, shaft and bearing arrangements are also required.'
I have been given lots of data too, such as engine data, propeller thrusts, the propeller and aircraft performance.
Jenni
This is a new project I have been given on my Mech Eng degree...I have to design a gearbox for a microlight...so if anyone has any suggestions or any help for me it would be great!Thanks!
'A speed reduction ratio of approximately 2.38 was originally specified for a microlight powerpack, however a new gearbox must be designed to accomodate a range of low speed high efficiency propellers. It is felt that the higher gear ratio now required may dictate the need to use an epicyclic gear set.
Complete a detail design of the gear set required for the application.
An outline scheme of the gear case, shaft and bearing arrangements are also required.'
I have been given lots of data too, such as engine data, propeller thrusts, the propeller and aircraft performance.
Jenni
jenni-leigh said:
Hello everyone!
This is a new project I have been given on my Mech Eng degree...I have to design a gearbox for a microlight...so if anyone has any suggestions or any help for me it would be great!Thanks!
'A speed reduction ratio of approximately 2.38 was originally specified for a microlight powerpack, however a new gearbox must be designed to accomodate a range of low speed high efficiency propellers. It is felt that the higher gear ratio now required may dictate the need to use an epicyclic gear set.
Complete a detail design of the gear set required for the application.
An outline scheme of the gear case, shaft and bearing arrangements are also required.'
I have been given lots of data too, such as engine data, propeller thrusts, the propeller and aircraft performance.
Jenni
The key to the problem is that weight has to be kept low. Additionally compactness is key. The solution also needs an epicyclic. It also sounds that 2.38:1 may be too low, but it may not.
What you need is a Ravigneaux gearset with sufficient torque capacity to handle prop inertial loads and engine torque (T=Iomega)+(Tengine), bearings should take Thrust from the helical gearset plus Thrust from the prop when the L10 value of the bearing is at a reasonable service life - say twice engine rebuild life (say 500 hours??). What you can do is use an off the shelf Ravigneaux to keep costs down - my suggestion is to use an off the shelf design such as one by Borg Warner - they supply most of the automatic trans manufacturers with components.
The key issue though would be that you do not want to use the auto trans system of pump and pressurised clutch engagement. That would lose pressure in manoevres and hence prop drive. Not good. I suggest having some sort of pre engagement that locks the gear elements in place in a preselectable manner before take off. The pilot could decide whether the weather dicated a slow prop speed or a fast one, then selected the speed before take off.
I'd look in the Bosch handbook for the Ravigneaux gear equations, you need a bearing supplier book for the sizing, I'd suggest a Timken or SKF book, the case needs to be made from a low pressure die casting aluminium alloy, I suggest LM25, when calculating thickness of ribs use minimum properties of yield stress LESS 10%. This is essential because the material properties are based on a cast test bar, which is easy to get right, put material in a mould and it will get porous, hence min less 10% is a good guide. Key is to package the gearset right, allow for thrust load, and a torsional load from the gear reaction. Additionally you need mounting lugs and a flange for input and output. If that was not enough, you need a lubricant level hole, I suggest you use sprash lubrication, a synthetic modified ATF using a GL5 friction pack. That is an EP lubricant for manual transmissions, so your gears will not fail.
You may not have everything in this, but there should be more than enough to produce a realistic design without glaring mistakes.
jenni-leigh said:
Hello everyone!
This is a new project I have been given on my Mech Eng degree...I have to design a gearbox for a microlight...so if anyone has any suggestions or any help for me it would be great!Thanks!
'A speed reduction ratio of approximately 2.38 was originally specified for a microlight powerpack, however a new gearbox must be designed to accomodate a range of low speed high efficiency propellers. It is felt that the higher gear ratio now required may dictate the need to use an epicyclic gear set.
Complete a detail design of the gear set required for the application.
An outline scheme of the gear case, shaft and bearing arrangements are also required.'
I have been given lots of data too, such as engine data, propeller thrusts, the propeller and aircraft performance.
Jenni
Excellent reply above.
I do this stuff for a living to, but with jet engines.
If we wanted a big reduction we wouldn't use an epicyclic. We'd use a steped reduction. In general a big one step reduction is non preferred, hence the epicyclic suggestion above. An alternative, which we'd use instead, is to have a multi-step reduction. i.e. to get 4:1 use two 2:1 stages. This is probably cheaper and simpler unless there is an off the shelf unit already existing.
Regarding bearings I'm fairly certain that you'd never find a standard bearing in a large aircraft engine transmission. I've been doing this for fifteen years and we always use special aerospace bearings. Standard bearings may be acceptable on small aircraft, I don't deal in that area so I can't say for certain.
The UK aero bearing industry is stuffed. Long ago it used to be Fafnir and RHP. Then they changed to Timken and Aero Engine Bearings, same companies, just the name changed. FAG (Germany) has hoovered up their business. The bearing catalogues are OK as a guideline but have some serious pitfalls for the uninitiated. Basically the life formula is rubbish and they know it but no one is prepared to say the emperor has no clothes. Just as an example a very high L10 life can indicate a low life in practice! Your lecturer is very unlikely to know this so use the books and they'll think you've done it properly.
Alternatively I could run the data through our (RR) software for you if you want.
Good Luck
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