• Our booking engine at tickets.railforums.co.uk (powered by TrainSplit) helps support the running of the forum with every ticket purchase! Find out more and ask any questions/give us feedback in this thread!

Wheel "quartering"

Status
Not open for further replies.

Flying Phil

Established Member
Joined
18 Apr 2016
Messages
1,924
On steam locomotives, with coupled driving wheels, the crank pins are arranged at 90 degrees side to side - "Quartering" so that the power strokes are evenly spaced (and the coupling rods do not bind?) with two or four cylinders. I am not sure what happens with three cylinder engines and especially when the centre cylinder is inclined relative to the outer cylinders (Bulleid pacifics etc.) are the three crank pins at 120 degrees?
 
Sponsor Post - registered members do not see these adverts; click here to register, or click here to log in
R

RailUK Forums

edwin_m

Veteran Member
Joined
21 Apr 2013
Messages
24,793
Location
Nottingham
I don't know about three cylinders, but the other reason for the 90 degree arrangement with two or four is that when one side is at top or bottom dead centre the other side will be exerting maximum torque on the wheels. Setting them at 180 degrees would result in an embarrassing lack of ability to start on occasions.
 
Last edited:

hexagon789

Veteran Member
Joined
2 Sep 2016
Messages
15,713
Location
Glasgow
There are two possible 'crank angle' arrangements with three cylinder steam locos. Either all the crank pins are set 120 degrees apart (by far the most common arrangement and used by Gresleys locos such as the A3s, A4s etc. Or the outside cranks are set 90 degrees apart and the inside at 135 degrees, this arrangement was used on some compound locos as in compound mode the exhaust beats are evenly spaced.
 

Cowley

Forum Staff
Staff Member
Global Moderator
Joined
15 Apr 2016
Messages
15,686
Location
Devon
Interesting question, from what I remember from listening to Bullied pacifics they have a beat that goes one two three, one two three (a driver pointed this out to me once). Reading the above posts then, would the Gresley ones be more even? I'm no expert and I hope this makes sense?
 

hexagon789

Veteran Member
Joined
2 Sep 2016
Messages
15,713
Location
Glasgow
It would appear that I've have slightly misinterpreted the data on Gresley Pacifics. While the use of Gresley valve gear necessitates setting the crank pins at 120 degrees apart on three cylinder locos, this only works if the inside or third cylinder is not inclined. As the inside cylinder on both Gresley Pacifics and Bulleid pacifics are inclined the crank pins are not aligned at 120:120:120 but rather at 120:113:127 in order to compensate for the incline. This still provides a distinct regular three-year exhaust though.
 

Flying Phil

Established Member
Joined
18 Apr 2016
Messages
1,924
Thanks Hexagon - so that means the coupling rods are not "Quartered" as such, on those locomotives?
 

Deepgreen

Established Member
Joined
12 Jun 2013
Messages
6,340
Location
Betchworth, Surrey
I don't know about three cylinders, but the other reason for the 90 degree arrangement with two or four is that when one side is at top or bottom dead centre the other side will be exerting maximum torque on the wheels. Setting them at 180 degrees would result in an embarrassing lack of ability to start on occasions.

Which can happen if the cylinders are at certain positions on stopping - I have seen locos that have had to set back a tad (a couple of feet) before proceeding.
 

edwin_m

Veteran Member
Joined
21 Apr 2013
Messages
24,793
Location
Nottingham
Which can happen if the cylinders are at certain positions on stopping - I have seen locos that have had to set back a tad (a couple of feet) before proceeding.

That can't be a top/bottom dead centre issue, as in either position the coupling rod is directly in line with the centre of the wheel so that cylinder can't turn the wheel in either direction. It's like what happens when a bicycle pedal is straight up and down, without the option of pushing it round with your foot to a more convenient position.

There must be something else going on here - perhaps setting back to slacken the couplings so the loco gets moving before it has to cope with the weight of the whole train? Other suggestions welcome...
 

markindurham

Member
Joined
1 Nov 2011
Messages
385
That can't be a top/bottom dead centre issue, as in either position the coupling rod is directly in line with the centre of the wheel so that cylinder can't turn the wheel in either direction. It's like what happens when a bicycle pedal is straight up and down, without the option of pushing it round with your foot to a more convenient position.

There must be something else going on here - perhaps setting back to slacken the couplings so the loco gets moving before it has to cope with the weight of the whole train? Other suggestions welcome...

Slightly incorrect valve settings might cause a failure to start from one particular spot. As for setting back - passenger stock would be quite tightly coupled anyway- you would get a tiny amount of buffer compression, but hardly enough to make a difference. Remember, coupling 'snatch' is pretty uncomfortable and can cause coupling failure, so it's not desirable. Of course, if the stock is buckeyed throughout, the train is effectively 'solid'.
 

Deepgreen

Established Member
Joined
12 Jun 2013
Messages
6,340
Location
Betchworth, Surrey
Slightly incorrect valve settings might cause a failure to start from one particular spot. As for setting back - passenger stock would be quite tightly coupled anyway- you would get a tiny amount of buffer compression, but hardly enough to make a difference. Remember, coupling 'snatch' is pretty uncomfortable and can cause coupling failure, so it's not desirable. Of course, if the stock is buckeyed throughout, the train is effectively 'solid'.

Indeed - I have seen the whole train set back a foot or two before being able to proceed, presumably in order to gain a tiny amount of momentum to overcome the 'wrong' stopping point problem.
 

Tomnick

Established Member
Joined
10 Jun 2005
Messages
5,826
That can't be a top/bottom dead centre issue, as in either position the coupling rod is directly in line with the centre of the wheel so that cylinder can't turn the wheel in either direction. It's like what happens when a bicycle pedal is straight up and down, without the option of pushing it round with your foot to a more convenient position.

There must be something else going on here - perhaps setting back to slacken the couplings so the loco gets moving before it has to cope with the weight of the whole train? Other suggestions welcome...
Possibly when struggling to start on a rising gradient, so setting back a few inches is the much easier option to get away from the dead spot?
 
Status
Not open for further replies.

Top