crankpin position

[sej]

Hi there;

 

Just a thought; why are the crankpins on a steam locomotive's wheels in the position that thy generally are, about half-way between the axle and the rim? I can understand that it would be rubbish for the rods to try to act on the axle centre but why not nearer the rims?

 

Cheers

Simon

[Castle]

Hi Simon,

 

Think about this - the further out towards the rim of the wheel you make the crank pin, the longer distance your piston has to travel per stroke. The position on the wheel has to be a balance between the lever action of the piston on the wheel and the fact that you don’t want really long cylinders! The longer the stroke, the more mass that is involved in the system, the faster the piston will have to travel per revolution of the wheel and the more ‘violent’ the forces are as a result.

 

A Castle has 6’8 1/2” diameter driving wheels on new tyres. If you move the crank pin out to say, 3’ from the centre, that means a piston travel of 6’ and therefore a clear bore in the cylinder of that length. This is without the end covers, add another 6” at least for that. Then you have to support the end of the piston throughout its travel and it needs to be at least as long as the piston travel. That’s over 12’ before you have added a connecting rod. Then imagine the forces that little lot would generate in motion.

 

To give you an idea, Castle cylinders are 16 in × 26 in (406 mm × 660 mm) that is bore x stroke. The crank pin is therefore 13” from the centre of the wheel - way below the half way estimate.

 

All the best,

 

Castle

Edited January 8, 2018 by Castle

[Miss Prism]

Reciprocating mass is the determinant. See: https://en.wikipedia.org/wiki/Engine_balance

[Castle]

Hi Miss P,

 

Just added that bit above - realised I hadn’t explained it well enough while you posted!

 

All the best,

 

Castle

[sej]

Thanks very much Castle and Miss P. It usually seems that things end up as a compromise; if piston length didn't matter would you get more power by having the crankpins near the rims? (I don't think I've worded that very well!).

 

Cheers

Simon

[Castle]

Try it out mathematically Simon:

 

https://en.m.wikipedia.org/wiki/Tractive_force

 

https://en.m.wikipedia.org/wiki/GWR_4073_Class

 

Top one will give you the tractive effort calculation (very easily used).

The bottom will let you try it out on Pendennis Castle!

 

All the best,

 

Castle

[Nick Holliday]

There is obviously a difference regarding this topic between inside and outside cylinder locos. With the latter, as discussed, the throw on the wheel will be around half the piston stroke length. Some engineers tried to keep the rods closer to the axle centre line, by forging crankpins with offset centres for the connecting rod and coupling rod, but the difference was only an inch or so.

With inside cylinders, there was no direct correlation between the cylinder stroke and the coupling rod throw. Members of the Brighton Circle are currently discussing this very topic, and it appears that Stroudley preferred to have a 9" throw for the rods, even though the cylinder stroke was around 24".

Apart from the consideration of the forces involved, an extended throw would also have required the splashers to be enlarged to accommodate the rods and their bosses, at least with those with flat running plates - not such a problem for a BR standard with the running plate well clear of the wheels!

[Miss Prism]

iirc, Adams liked 9" throws as well.

[Miss Prism]

if piston length didn't matter would you get more power by having the crankpins near the rims?

 

Piston speed was another engineering compromise area. (Lubrication and wear, etc.)

[LMS2968]

When a designer starts on a new loco, he will start with some major dimensions: wheel diameter. cylinder bore and piston stroke. He doesn't consider the crankpin's position in the wheel as such, as this is entirely a function of the stroke: the centre to centre distance of the wheel and crankpin will ALWAYS be half the stroke.

 

It isn't true to say that the distance was half way to the rim: Stanier's Big Lizzies and 8Fs both shared a common 28" stroke so crank length equals 14 inches. The first had wheels 6' 9" diameter; the second 4' 8 1/2".

 

Moving the crankpin outwards and increasing the stroke will give more torque, but also higher piston speed. GWR engines usually had a 30" stroke; Bulleid Pacifics 24", i.e. pin distance 12" so just under a third of 6' 2" wheels.

[rodent279]

Effectively, the position of the crankpin relative to the wheel diameter determines the "gearing" of the loco, for want of a better phrase. A 30" piston stroke, with a 15" crankpin radius, on say 4' 7" wheels, will develop it's maximum torque, and therefore tractive effort, at a lower speed than the same piston stroke on an engine with larger wheels. That's why freight engines had smaller wheels-they are not expected to travel as fast in the first place, and starting a heavy freight train, or slogging uphill with it, requires all the torque to be developed at low road speeds. It also has a bearing on the maximum efficiency, as the faster the piston speed, the faster the steam consumption rate.

The mean piston speed for a freight engine like an 8F will be around the same at 25-30mph as a Duchess at more like 70mph.

Edited January 8, 2018 by rodent279

[rodent279]

A quick calculation tells me that a Duchess had the same mean piston speed at 45mph as an 8F at 30mph.

If you compare the closely related GWR 28XX, with 4'7" wheels and 2 X 18"X 30" cyls to a Saint, with the same cyl dimensions but 6' 8" wheels, the numbers come out very similar, and I would expect a Saint at 75mph to be consuming about the same amount of steam as a 28XX at 50mph, in theory at least.

[Miss Prism]

Brunel had very stringent ideas about piston speed. It drove his loco subcontractors mad...

[PenrithBeacon]

When a designer starts on a new loco, he will start with some major dimensions: wheel diameter. cylinder bore and piston stroke. He doesn't consider the crankpin's position in the wheel as such, as this is entirely a function of the stroke: the centre to centre distance of the wheel and crankpin will ALWAYS be half the stroke.

 

It isn't true to say that the distance was half way to the rim: Stanier's Big Lizzies and 8Fs both shared a common 28" stroke so crank length equals 14 inches. The first had wheels 6' 9" diameter; the second 4' 8 1/2".

 

Moving the crankpin outwards and increasing the stroke will give more torque, but also higher piston speed. GWR engines usually had a 30" stroke; Bulleid Pacifics 24", i.e. pin distance 12" so just under a third of 6' 2" wheels.

Beg to differ, but only I suppose in detail.

 

The designer of a loco will start with a requirement from the traffic department after approval from the board. He (always a he) will need to know the expected weight of the train, it's expected end-to-end speeds and any weight restrictions en-route. From this a decision will be made concerning the number of cylinders, two, three or four. If its three or four that will have an effect on the cylinder stroke because of concerns about the reliability of inside crank big ends. No British engine with inside big ends had a cylinder stroke more than 28" and that was only one (IIRC) ie DoG which equalled this figure. Due to loading gauge restrictions in Britain a designer could get no more power from four cylinders as he could from three and it was generally perceptions of balancing that dictated whether a designer used three or four. The use of four implied a greater maintenance risk and costs.

 

Churchward started the use of 30" stroke on the GWR for his larger two cylinder engines but he was content with 28 or 24 for smaller engines (inside or outside cylinders).

 

There's a lot more to this than might meet the eye, but that will do for now

[bike2steam]

Thanks very much Castle and Miss P. It usually seems that things end up as a compromise; if piston length didn't matter would you get more power by having the crankpins near the rims? (I don't think I've worded that very well!).

 

Cheers

Simon

 

You'd  need more steam, bigger boiler, and knackered fireman (firemen !?).  

[rodent279]

would you get more power by having the crankpins near the rims?

You'd need more steam, bigger boiler, and knackered fireman (firemen !?).

I think the short answer to that question is no. Putting the crankpin near the rims means a longer stroke. On a loco with large wheels like the 6'8" of the GWR Castles would mean a stroke of 40". Technically possible, but it would mean a large reciprocating and rotating mass acting at a considerable distance from the wheel centre, which will produce greater centrifugal and fore/aft forces acting about the crank. This can be countered using balance weights, but not fully. Also, such forces, especially at speed, will increase the tendency of the wheels to lift off the rails, which increases the hammer blow on the track. Edited January 9, 2018 by rodent279

[IWCR]

Another  constraint  is  the  size  of  the  big  end  assembly,  this  has  to  fit  over  the  crankpin  but  still  have  enough  room  under  it  to  clear  track  level  obstructions  when  at  bottom  centre.  For  inside  cylinder  engines  it  also  has  to  clear  the  firebox  front  when  at  back  centre.  Not  to  much  of  a  problem  for  a  large  wheel  outside cylinder engine  but  a  definite  limiting  factor  for  a  small  wheel  inside  cylinder  loco.

 

Pete

[bike2steam]

I think the short answer to that question is no. Putting the crankpin near the rims means a longer stroke. On a loco with large wheels like the 6'8" of the GWR Castles would mean a stroke of 40". Technically possible, but it would mean a large reciprocating and rotating mass acting at a considerable distance from the wheel centre, which will produce greater centrifugal and fore/aft forces acting about the crank. This can be countered using balance weights, but not fully. Also, such forces, especially at speed, will increase the tendency of the wheels to lift off the rails at speed, which increases the hammer blow on the track.

Yep, just think of the 'hammer-blow'. A Bulleid pacific is supposed ( in theory) to be 'balanced', but in 1988 when a rather worn out 34105 was visiting it's namesake for the weekend, you could actually feel her, through the ground, a good half a mile away. Just think what a loco with a piston stroke of over 60 inches would be like.   

[sej]

Fantastic food for thought! Thankyou everybody.

 

Cheers

Simon

[AndyID]

Just a thought; why are the crankpins on a steam locomotive's wheels in the position that thy generally are, about half-way between the axle and the rim? I can understand that it would be rubbish for the rods to try to act on the axle centre but why not nearer the rims?

 

Sometimes they are quite close to the rims. Small 0-4-0 dock shunters are like that. They produce lots of tractive effort (torque) but their speed is limited.

[LMS2968]

Just to add a bit here, it is generally assumed that the crankpin position was the same across all coupled wheels. Not so. In an attempt to reduce the centrifugal forces generated by the coupling rods, there was a fad in the earlier years of the twentieth century to reduce their stroke by one inch. This meant that the driving crankpin was stepped, the coupling rod centred at, say 13.5" and the crankshaft portion at 14".

[kevinlms]

Beg to differ, but only I suppose in detail.

 

The designer of a loco will start with a requirement from the traffic department after approval from the board. He (always a he) will need to know the expected weight of the train, it's expected end-to-end speeds and any weight restrictions en-route. From this a decision will be made concerning the number of cylinders, two, three or four. If its three or four that will have an effect on the cylinder stroke because of concerns about the reliability of inside crank big ends. No British engine with inside big ends had a cylinder stroke more than 28" and that was only one (IIRC) ie DoG which equalled this figure. Due to loading gauge restrictions in Britain a designer could get no more power from four cylinders as he could from three and it was generally perceptions of balancing that dictated whether a designer used three or four. The use of four implied a greater maintenance risk and costs.

 

Churchward started the use of 30" stroke on the GWR for his larger two cylinder engines but he was content with 28 or 24 for smaller engines (inside or outside cylinders).

 

There's a lot more to this than might meet the eye, but that will do for now

But history books are filled with plans of locomotives, that were never built for various reasons.

 

Fowler for instance, came up with a design for a compound pacific, which fortunately never got beyond the drawing book.

 

Another classic example that gets discussed many times on RMweb, is a BR Standard 4-8-2 (at the presumed expense of an equivalent number of 9F's).

[jim.snowdon]

Just to add a bit here, it is generally assumed that the crankpin position was the same across all coupled wheels. Not so. In an attempt to reduce the centrifugal forces generated by the coupling rods, there was a fad in the earlier years of the twentieth century to reduce their stroke by one inch. This meant that the driving crankpin was stepped, the coupling rod centred at, say 13.5" and the crankshaft portion at 14".

I was aware of that in respect of the Ivatt GNR Atlantics, and by implication, the LBSCR clones, but how widespread was it? What does appear to be more common on inside cylinder locomotives is the coupling rod throw being smaller than the crankshaft throw. The reasons are logical enough, as a means for reducing the vertical bending forces on the coupling rods. Being part of the unsprung mass, there is a design conflict between keeping the mass down whilst retaining sufficient vertical and horizontal stiffness to avoid buckling.

 

Jim

[AndyID]

The reasons are logical enough, as a means for reducing the vertical bending forces on the coupling rods. Being part of the unsprung mass, there is a design conflict between keeping the mass down whilst retaining sufficient vertical and horizontal stiffness to avoid buckling.

 

 

Hi Jim,

 

Decreasing the coupling rod throw actually increases the tensile and compressive force on the rods. Also, because the ends are pivoted there is almost no vertical load. Unless the bearings seize they buckle due to compressive force.

 

Regards,

Andy