Stroke

[David HaynesParticipant @davidhaynes53962]

Simple question, when does the stroke of an inside motion differ from the stroke off the wheels? I suppose there is the matter of mechanical advantage that is applied for different loco uses, but I just wondered if there was any more thinking behind it.

Dave

Edited By David Haynes on 10/02/2012 19:08:55

[David HaynesParticipant @davidhaynes53962]

Parity between inside motion and wheels

[Sub MandrelParticipant @submandrel]

I assume you are comparing the stroke of inside and outside cylinders on 3/4 cylinder locos?

 

If they are the same bore and pressure, then all cylinders should have the same stroke.

 

I can see that an inside cylinder could be smaller bore than outside if it had longer stroke to compensate. I can also see that there could be some careful calculations around compund cylinders..

 

If you are comparing the stroke of inside cylinders with outside connecting rods, then there is no reason why they should be the same, aside from similar stroke allows bearings to be the same size.

 

Neil

[David HaynesParticipant @davidhaynes53962]

Thanks Neil. Yes this is in 3/4″ scale, but I would have thought the issue would apply to all scales. To clarify, I am comparing the stroke of the inside piston with the ‘stroke’ of the coupling rods, i.e. the offset from the crank pin to the wheel centre. The prototype drawing states ‘Cylinders 1′ 7 1/2″ Dia. 2′ 2″ Stroke’. This statement clearly refers to the cylinder stroke, but at no other point on the drawing, a well detailed G.A., is the driving wheel crank offset from the axle referred to. Scaling off the drawing, the offset from the c/c of the driving axle to the c/c of the big end journal is 13″. So far so good, as this would give a stroke of 2’2″ as indicated. However, using the same scaling, the c/c of coupling rod journal to the c/c of the associated axle, the distance comes out as 11 1/8″. clearly not the same as the piston stroke. Of the few 3/4″ scale drawings of inside motion I have access to, all have the same dimension for stroke and wheel crank offset. I wonder why this is?

Dave

[MICHAEL WILLIAMSParticipant @michaelwilliams41215]

Hi David ,

 

This is one of those rare things in engineering where there is in most cases no science involved at all . Most dimensions on a locomotive were basically sized by a mixture of copying what’s gone before , practicality and the need to usually cram big things into inadequate spaces . Some of the better locomotive engineers certainly did a few sums now and then but basically always came across the hurdle that there is only a limited range of cylinder bores and strokes and crank throws that are feasible for a British locomotive .

 

The ratio of inside crank throw versus outside coupling rod crank throw is almost always somewhere near 1:1 (with odd exceptions such as on some LNW engines ) . Nine times out of ten the stroke of the cylinders was set by best guess as to what was needed or just by established practice and the throw of the coupling rod cranks set by what ever wheel patterns where available .

 

A more interesting thing to look at is the angular disposition of inside cranks relative to outside cranks and the related subject of balancing .

 

MikeW

[David HaynesParticipant @davidhaynes53962]

Thanks for the comments.

The sectional plan shows that when the right hand (inside) connecting rod is a back dead centre, the right hand coupling rod is at front dead centre. At this position, the left hand connecting rod is at top dead centre but the left coupling rod is not shown; by deduction it will be at bottom dead centre. On the subject of balancing, the driving wheel balance weight is ahead of the crank pin slightly, on the left hand side it is about 35 degrees ahead of the crank pin. Are the left hand balance weights a mirror of the right hand? In this case, would the right hand balance be 35 degrees behind the crank pin?

Thanks again

Dave

[MICHAEL WILLIAMSParticipant @michaelwilliams41215]

Hi Dave ,

 

No quick answer to your question – it depends on the whole configuration of the engine .

 

If you could tell me the prototype or published design you are building I’ll see if I can work it out for you .

 

MW

[mgjParticipant @mgj]

I would suspect that somewhere a compromise is being made, possibly to do with space available.

 

Because what has not been mentioned is torque. By having uneven throws (without a corresponding correction in cylinder area), or not having them identical, for any given chest pressure, the deliverered torque is uneven. Thats not desirable since it tends to lead to vibration and whip in the crank axle/shaft.

 

Con rods don’t have to be identical length, because they just connect two points, but longer is generally better, at the cost of rigidity. You’ll get differences in accelerations and  angular velocity in the bearings, and extreme differences will cause vibration, (none of which are good things) and losses because of angularity, but in principle, but as long as you connect the bit that pushes with the bit that turns it should work.

Edited By mgj on 12/02/2012 08:42:56

Edited By mgj on 12/02/2012 08:48:47

[David HaynesParticipant @davidhaynes53962]

I have just looked at Nick Feast’s Q1. He has the crank axle offset from c/c as 19mm and yet the wheel cranks are strangely 20mm from c/c.

 

Dave

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