One Response to “Propeller Shaft”

1. cands589
   November 10th, 2011 @ 10:29 pm

   You probably know the propellor shaft is the link from the gearbox to the wheels, and this goes round to make the wheels go round. The speed is therefore going to be in revolutions per minute.
   Your problem here is that you don't know how many times it goes round for each time the engine goes round or each time the wheels go round – this depends on the design of the car.
   If you knew the ratio of the gearbox (how many times the propellor shaft goes round for each rotation of the engine) in second gear, you could answer this exactly, but you don't have this information.
   You can make some estimates however.
   The simplest way is to estimate how fast the car is probably going at this engine speed. eg in second gear your speed is probably around 10mph. (This is only approximate – of course it depends on the car, and older cars tended to go faster for a given engine speed. It won't be 1 mph or 100mph)
   Now how large are the car's wheels? Say they are 50cm across. This gives a circumference of around 3 x 50 = 150cm
   So how many times do the wheels go round each hour. They travel 10 miles (as car speed is 10mph), which is about 20km or 20000 metres. Since the wheel has a circumference of 150cm = 1.5 metres, then the wheels go round 20000 / 1.5 = 13333 times each hour.
   So each minute they go round only 1/60 of this which is 13333 / 60 which is about 200 times.
   So the wheels are going round at 200rpm, but what about the propellor shaft. Again, you need to estimate the ratio of the gears which connect the propellor shaft to the wheels. These are probably not going to be that different in size – ie the propellor shaft speed will be similar to the wheels.
   So you can estimate that the propellor shaft is going at around 200rpm. But you should note this is an order of magnitude only. ie it won't be 20 or 2000rpm.
   Hope that helps you.