Steam & Excursion > Steam Loco question

My question is concerning the cylinders and rods on a steam locomotive.   If you have a two cylinder steam engine, is one piston at top dead center and the opposite cylinder completely open?  and if so, are the rods on each side of the locomotive opposite?  

Pardon my ignorance.   I was watching steam videos and trying to figure out how the "chuffing" matches up with the cylinders and rods.

Dave

On a two cylinder steam locomotives, the crankpins are set 90 degrees from one another.  In other words, if the rods on one side are at bottom, the other side with be either at 3 or 9 o'clock, depending upon the railroad's practice.

Remember that steam locomotive cylinders are double-acting, i.e. there is steam supplied to BOTH sides of each piston. That results in 4 exhausts per complete revolution on a two cylinder steam locomotive, with the exhaust, or "chuff", every 90 degrees of rotation.

Good questions - a good place to start learning about these fascinating machines.

In addition to what Frisco1522 wrote, let me add that there is no top dead center nor bottom dead center.  Instead, dead centering occurs at th 3 o'clock and 9 o'clock positions.  Setting the left and right drivers 90 degrees apart assures that only one side, rather than both sides at the same time, of the engine can ever be dead centered.

Since there are four power strokes for each revolution of the drivers and they are set 90 degrees apart, there are four "chuffs" for each revolution, for 3, 6, 9, and 12 o'clock.  Not only does this largely eliminate dead centering, but also gives more even power distribution and use of steam.  (Not actually at those exact angles - just as for automotive cylinders, steam cylinders timeing is slightly off.  The chuff is slightly before the 3, 6, 9 ,12 positions to allow time for complete exhausting of the spent steam, and the engineers controls allow him to vary this timing as speed increases or decreases.

More questions?  Ask!  Several people on this board have been qualified to work on and run steam locomotives, and we like to teach.

-Larry Doyle



Edited 2 time(s). Last edit at 12/13/20 08:14 by LarryDoyle.

I once heard that Pennsy steam engines were right lead, while most others were left lead.  Can someone like Wes or HotWater or anyone else who knows comment on if this is correct or not?

Keystone1 Wrote:
-------------------------------------------------------
> I once heard that Pennsy steam engines were right
> lead, while most others were left lead.  Can
> someone like Wes or HotWater or anyone else who
> knows comment on if this is correct or not?

I seem to remember just the opposite, i.e. PRR was lefthand lead, while the rest of the U.S. and Canada was righthand lead.

Ok......maybe you are correct HotWater.....but at least we know what each other is talking about.   Some people have no clue.  Thanks for replying to this esoteric topic.

So, what is lefthand lead as opposed to righthand lead in a steam locomotive.  Need some educating please.

Greg

On just about all US RRs, when the
right-side piston is all the way forward,
the left-side piston is midway thru its
stroke, with the left-side crankpin at
12 o'clock. That's what people mean
by "right-hand lead".

Don't recall how SP's cab-forwards
were set.

Thanks for the explanation. 

Greg

OK..
You'd have to be standing on the engineer's side of the engine looking through the
frame and drivers, to the other side.

For covention's sake, we're going to be talking about thre driver wheels as if they were 
an analog clock face: with 12 o'clock at the very top of the driver, and 6 o'clock would be the
very bottom of the crank circle.  3 o'clock would have the piston rod on that side fully buried in
the cylinder, at Front Dead-center.  The 9 o'clock position would have the crankpin at the rear 
of its crank-circle, and the piston rod would be fully exposed, outside the cylinder casting,
at its rear dead-center.*

Now, you have the "engine driver-person" move the engine slowly forward until the
crank-pin in front of you is at the 3 o'clock position.

The crankpin on the fireman's side will NOW  be at that wheel's highest position: 12 o'clock.

Since 3 o'clock is 'ahead' of 12 o'clock, it is said to be a 'right-lead' engine.
On most PRR locos, with the engineer's side crank-pin at the 3 o'clock position, the 
crankpin on the fireman's side would be Down at its 6 o'clock position...
6 o'clock is ''ahead'  of 3 o'clock, so the PRR locos with that arrangement would be 
called left-lead engines.

The counterweights on the drivers are virtually on the same half of the drive rim, but off-set
by 90-degrees... With thousands of right-lead locos, that inherent imbalance pounded the roadbed 
edges ( where the ballast rocks are least supported), and meant that track repair crews couldn't
keep up with the many miles of sagging and pounded-out edges of the R-O-W..    

PRR chose to operate their locos so that the imbalance 
of the driver counterweights were on the left rail, of a multi-track mainline .... the  ties
and ballast rocks. had better support and would not get pounded-out as badly as the  outer rail...
from the inherent im-balance of the two counterweights on  one half of the crank circle.
Thats why the PRR tried to standardize on the left-lead arrangement.

Now, the 'chugs' thing..

The double-acting pistons have steam admitted at the beginning of each power stroke.
Double-acting means that there is a pressure head at each end of the cylinder where the piston
is confined.   The front head is a virtual disk enclosing the entire front of the cylinder.

The rear cylinder head has the piston rod reaching out through the hole in the rear head.  
The hole is a sealing chamber that has suitable sealing elements that allow the piston to slide
freely, at high speeds, without leaking any steam.

At each end of the cylinder (top) is a semi-circular opening from the upper (valve) chamber.
These ports are at each end of the cylinder, and the piston face  is always in communication with
the port at its nearest end.  The port carries the pressurized steam to the piston faces, while the 
piston face on the OTHER SIDE of the piston, is exhausting it's 'used' steam ....up the very port that
had been carrying live steam for the "power" stroke.

As has been said, the double sided piston makes for two power stokes, per complete driver
revolution..

Since there are two cylinders, one lagging the other by 90-degrees, there will be 4 -exhaust chugs,
per complete driver revolution.,...each chugg will be 90-degrees apart.

The steam pressure/exhaust timing is controlled by a 'spool-valve' in the upper chamber, above
the piston and its cylinder.   The two lobes of the spool valves cover a circular ring of ports
( at each end of the valve bushing)  which  is the entrance to the  port/passage that leads down
to the piston faces at each end of the cylinder.  

The valve is timed so that it admits steam to the nearest piston face when the piston is
at the end of its forward travel...  The space between the two 'lobes' of the spool valve are
always pressureized when the engineer opens the throttle.

The spool valve is moved by the "valve gear" that is oscilllated by the reciprocal motion
of the driving rods and levers.  The valve always moves when the drivers move.  
But at a slightly variable rate, controlled by the engineer..

With the spool in the forward position, it admits steam to the front face of the power piston.

Very neatly!  Tha spool lobe at the other end of the spool valve, simultaneously opens the
steam port ( in the valve bushing) to the exhaust nozzle and stack.  
That opened port is the 'chugg' you hear.

it occurs at the same time as the high pressure steam is sent to the front face of the piston,
for the next 180 degrees of the power stroke.   At the end if the that power stroke,
the rear steam posit is pressurized with live steam, with the driver crank going over the top of the
driver crank circle .... At 359-degrees, the cycle is complete, two power strokes, and two exhsust
chuggs.

But, there is the whole "mating cylinder" and valve on the fireman's side ... 
The two sides, exhausting in sequence is what give the 4-chuggs per revolution.

However... the dimensions are very large, and the angles & levers involved make for
inherent, unavoidable dimensional variances.  Those variances you can hear in a very 
'lame' engine. A lame engine will have a more-pronounced, louder CHUGG, than the next three
chuggs.   ( Origin of the famous line:  I THINK I can, I THINK I can ...etc..)

There was a time when a very few "valve-setter" machinists could make very precise
alterations in the the valve gear levers' dimensions, so that the unavoidable (angular) errors
were minimized, and ALL the exhaust 'chugs' were pereceptably indenticsl in intensity....
From both sides of the engine..... Today, that fine- tuning ability is truly a LOST ART!

With a precisely timed engine, there was NO apparent "lame-ness" in the exhaust beats,
even though the engineer may vary the valve cut-offs of the power strokes.

An engine that has perfectly-even, well timed exhaust chugs, is said to be "perfectly square"
In machinist's language, the engine would sound "as square as a die"!

[ Extra Credit Question:  Whem powering:  Across the bottom of the driver crank circle, what's moving
the train: The Piston, or the Front Cylinder head?  Explain.)

​W.

( * Dead Center:  the point in the piston travel where any further movement is
ALWAYS away from the nearest cylinder head) 

 (To be proofed, yet)...


 



Edited 11 time(s). Last edit at 12/13/20 12:50 by wcamp1472.

timz Wrote:
-------------------------------------------------------

> Don't recall how SP's cab-forwards
> were set.

Running any engine backwards (or redefining it as a cab-forward) does not change its left-lead or right-lead status.  If its a right lead engine, it's still right lead with respect to its direction of movement.

-LD



Edited 1 time(s). Last edit at 12/13/20 14:24 by LarryDoyle.

Thanks for the responses!  It really explains everything clearly.

Dave

OMG.......what did I start.     Thanks Wes .  And HotWater too.         If anyone is on a game show, the question might be.....is a Pennsylvania steam locomotive a right or left hand lead.

Wes......some guys have turned in shorter papers for their PHD !

Tha answer is:.....
"It depends..."

If the PRR J1s --- war Babies--  were 'Allowed copies' ( by the WPB)  
of the C&O 2-10-4s, were the PRR J1 engines built as Right or Left lead?

( I've noted that my collection of various Brass PRR models, 'O' & 'HO',  
are all right-lead: incorrect )..

I don't have a model J1...

​W.

 

Brevity..

( in response to "PHD theses"...remark, above )

At private School* I attended, one instructor graded his term papers by
throwing the essays down the Entrance Hall flight of stairs.

The heavier papers landed first and the lighter papers went drifting
to the lower steps.  The heavy papers got lower marks than the lighter
papers --- that floated further down the stair steps. 
Not too many kids got 'A'.  

My wife, the English teacher, on the verbose instructions for the essay question .....
.. embedded a phrase:  "If you read this, and are first  to turn your paper "in",
you are excused , with an 'A' for the grading period" !

Many of her students were dismayed .... typically, the ones following her instructions,
were the 'A' students , any how. 

[Back in the 1940s, I was a Poster Boy for the ADD-kid...before Ritalin, etc..]

​W.

* The Pennington School
.



Edited 1 time(s). Last edit at 12/13/20 14:42 by wcamp1472.

Another question about pistons and rods. Hopefully not hijacking this thread too much...
How is it determined where to set the crank pin on the dirver wheels. Not in terms of timing, but at what radius? I know piston stroke is a big factor, but is there a common rule as to  how to do this?  Seems to me, you get higher "gearing" closer to the axle.  We also see engines destined for faster serrvice tend to have larger wheels. Can't you do much of the same by changing the crank pin radius position?   Insight about this is more than welcome.
 

It’s a KEY CONCEPT...

The main consideration is limiting
the speed of the piston on the cylinder walls, as well as ‘mechanical leverage’ advantage
at the rail.

Switch engines and ‘drag freight’
Locos have the crankpin all the way
down at the driver rim.

Higher speed locos move the crankpin closer to the axle center.

It’s a matter of leverage between the axle center, the crankpin and the rail.... together, the 3 points make a lever: with the rail as the fulcrum, the main rod connection at the crankpin is the force-point, and the axle/ driving box is the ‘weight’ being moved.

The mechanical advantage varies
from zero ( at the front and rear
‘dead centers’ —- of the piston travel), to the maximum leverage with the crank at top, or bottom of its crank circle.

It’s comparable to the concept of
Automobile differentials with varying
Ratios: like a 3.09 rear-end vs. a 4.11 rear-end.

A good rule of thumb is that freight engines have about a 50% position
of the crank ( between the axle center and the rail), passenger locos probably 40% to 33%, and switch engines have the crank at virtually right-on the wheel’s rim.

On Shay-geared locos, You can also have fun calculating the mechanical advantage of the wheel gears in relation to the
Shaft -driven pinion gears.
( count the teeth on the pinion
and the teeth on the wheel-gear,
to get that equivalent ‘rear-end’
ratio.. ).

W.

(diesel-electric locos typically
use a 4:1 gear ratio at the pinion/wheel gear on the axle. The pinion is mounted on the armature shaft, and axle gear drives the wheelset. But this ratio is governed
by the maximum axle RPMs —- loco speed—-of the armature, before it flies apart. See also the driver diameter on Amtrak’s early
AEM-7 Electric locos... you’d be surprised..)

Posted from iPhone



Edited 3 time(s). Last edit at 12/14/20 04:32 by wcamp1472.

> I know piston stroke is a big factor, but...

You know piston stroke is the factor -- right?
If stroke is 32 inches, crankpin radius has to
be 16 inches.

So why do some engines have 32-inch
stroke and other similar engines have
34-inch? No one here knows.