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Overdriven driveline

Running the gearbox and drive train faster than engine speed
  [vote for,

A gearbox and drive train running faster than the engine needs to handle less torque, and may therefore be of lighter construction.

This is the opposite of motorcycle practice, where the clutch and gearbox often turn at something like two-thirds engine speed. But then motorcycle engines tend to be low-torque, high-speed units.

What I have in mind is a rwd application where drive taken from the clutch is stepped up to, say, twice engine speed by a set of gears. The gearbox, propeller shaft, and final drive all then operate at twice the speed it would without the step-up gearing. The torque capacity of all these components needs to be half of what would conventionally be required. If there are inboard rear brakes, small drums would do the job of large discs. Drive is stepped down at the rear wheels by inverted portal hubs.

What I see in my head is Unimog portals upside-down on a DeDion axle, used in a street rod.

The main advantage is that it would allow more efficient packaging in a rwd car by allowing a smaller drive train mounted lower in the vehicle. There may be weight savings, though they are likely to be offset somewhat by the extra three gear sets and attendant casings, etc.

The main problems I see are developing synchromesh capable of dealing with the rotational speeds. Oil drag might be a factor. One might design with relatively thin oil in mind. Obviously the propeller shaft would be more sensitive to imbalance and would have to be located very securely (perhaps ruling out live axles) but it would be possible to make it lighter.

Ned_Ludd, Jan 18 2007


       A few flaws in your cunning plan:   

       1) Most motorcycle clutches run at engine speed. Note also that by definition, the gearbox input runs at one speed, the output at another, thats the point so how can you have a gearbox running at engine speed, it would nbot be "gearing" anything, it would have a ration of 1:1 !!!   

       2) Drivetrain components are run at lower speeds because at a lower speed there is:   

       a) Less heat produced b) Less wear c) Safer operation d) Less likely to do major damage if a catostrophic breakage occurs e) quieter   

       3) Running large portions of the drivetrain at higher speeds and higher revving engines is actually what makes many performance motorcycle engines wear out sooner than their car counterparts.   

       You would not be able to make it smaller and lighter as the components would still have to witstand increased operating wear and higher running temperatures.   

       If you have hub gearboxes such that the speed to the wheels is stepped down again you would be increasing the complexity and introducing more friction and hence making the system less efficient. Either way, you lose.   

webfishrune, Jan 18 2007

       I think that the step-down gearboxes would be likely to consume any weight advantage you would have gained. In engineering terms it is feasible, but the complexity is too much to pay for the advantages. Plus what [webfishrune] said - increased heat generation and wear.
david_scothern, Jan 18 2007

       I changed the second paragraph to reflect what I mean more accurately. But I think you knew what that was.   

       There are two developments that might be more practical:   

       1. In factory applications, using an all-indirect gearbox of which the highest ratio is in the order of twice engine speed, and   

       2. In cobbled applications, running the step-up behind a conventional gearbox, simply to lower the driveline and reduce the size of the final drive.
Ned_Ludd, Jan 19 2007

       //needs to handle less torque, and may therefore be of lighter construction//   

       However they must be of correspondingly higher precision to remain in balance at these high speeds. For a car engine this could be over 12000 rpms, all the gearing will need to be of higher precision adding significant cost to their production. They would be correspondingly less durable to adverse conditions.
jhomrighaus, Jan 19 2007

       The systems I work on are designed (by better engineers than I) to run at 18,000rpm for 30,000 hours, transmitting 500kW through an 80mm dia shaft whose wall thickness is just 2mm.   

       Having said that, those gearboxes cost the thick end of £100,000/$150k.
david_scothern, Jan 19 2007

       I did not mean to imply that such numbers are not possible or reliable, but in the application that has been submitted here they would be subject to many other forces than just harmonic vibration from their rotation. For a vehicular application durability is key and the ability to deal with adverse conditions. Such a shaft being hit by a stray rock or being soaked in salt spray for 5 years would cause it to shred itself if fairly short order.   

       F1 racing engines regularly see those kinds of numbers but as you indicated cost becomes a significant deterrent to even moderate usage for a rather limited benefit.
jhomrighaus, Jan 19 2007


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