Question: Rule of thumb - driver diameter vs. speed

[NGT6 1315]

Afternoon,

 

just as a quick question: I remember there was a rule how there would be a relationship between driver diameter on a steam engine and its maximum speed. However, was it "driver size in inches equals maximum speed in miles per hour" or was there another factor involved?

 

Thanks in advance .

[beast66606]

Afternoon,

 

just as a quick question: I remember there was a rule how there would be a relationship between driver diameter on a steam engine and its maximum speed. However, was it "driver size in inches equals maximum speed in miles per hour" or was there another factor involved?

 

Thanks in advance .

 

From my basic knowledge of physics I would say there are lots of other factors - amount of steam the boiler can generate, mechanical rolling resistance, wind resistance etc. - all go to limit the maximum speed.

[PhilH]

Not really - an example is the BR 9F 2-10-0 which were occasionally pressed into passenger service on the WR and ER. Although they only have 5 foot diameter driving wheels they used to run at speeds well into the seventies on these trains. What is also reckoned to be a limiting factor on sustained high speed is the ability of the boiler to supply steam for these periods and also the design of the steam passages, the exhaust passages in particular as once the steam has done it's job getting it away from the cylinders fast enough can be a problem.

[BR60103]

Yes, driver diameter in inches = maximum miles per hour is a rule of thumb. But only that.

When some BR people saw the 9Fs beting along in passenger service, they proposed exactly that speed limit for them, until it was pointed out what that would do to the A3 and A4 classes (and equivalents). (At least, I read that somewhere)

[NGT6 1315]

Thank you all, guys . I am aware that there are several factors contributing to how fast a steamer can go, of course.

 

I also read that, as a general rule, the drivers on a steam engine should not exceed about 350 rpm regardless of their size, which has mostly to do with unbalanced mass on the drivers, motion and cylinders.

[jamie92208]

Another factor was piston speed as the piston has to reverse twice for every revolution of the wheels. This was also related to the balancing. As stated above there were many factors involved as engines that were coasting downhill were able to go faster than normal as they didn't need to get rid of exhasut steam through often tortuous steam passages and poorly designed valves. However as the cylinder lubrication in superheated engines depended on there being a small amount of atomised oil in the inlet steam the regulator was normally left slightly open to make sure that the oil got through.

 

Jamie