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Cylinder Erection

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David Haynes28/07/2010 01:07:55
168 forum posts
26 photos
I am at the stage of erecting cylinders on a simple 3 1/2" gauge loco and am uncertain if the setup is correct.  I want to be certain before drilling for frame fixing bolts. on the first cylinder.  I am fairly happy that the components are all correctly dimensioned but it seems stiff.  All I have fitted is the piston and rod in the cylinder, front and rear cylinder covers, guide bar, crosshead and connecting rod.  Yet when I turn the wheels it feels stiff and there is a bit of jolt as the wheel reaches front dead centre (jumping from one side of dead centre to the other),  This does not happen when the wheel reaches back dead centre where the movement is smooth.  The axles have been fixed at the correct operating height and I have obtained alignment of the cylinder with the centre of the driving wheel by use of a pointed sleeve over the piston rod before fixing the crosshead.  The distance from the centre of the driving wheel to the cylinder has been worked out from the drawings.  The piston has packing yarn fitted, which will make things stiffer but I don't know if this is too stiff or not.  Please can someone advise.  Thanks, Dave
John Baguley28/07/2010 02:21:33
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517 forum posts
57 photos
Hi Dave,
 
The 'jolt' at front dead centre could be the piston hitting the front cover. Have you tried it with the front cover off? If the piston is hitting the cover and you are happy that the cylinder is in the correct position, you may need to shorten the piston rod slightly to equalise the clearance at each end of the cylinder.
 
Is the piston free to move in the cylinder and slidebar assembly before you fit it to the frames ?
 
I don't use the graphite packing (use PTFE rings instead) but it is possible you've made it a bit tight which could be part of the problem. I think you're supposed to pack the groove and then roll the piston on a flat surface to squeeze the packing down until it is more or less level with the surface of the piston.
 

 John

David Haynes28/07/2010 08:37:08
168 forum posts
26 photos
Thanks for that John, you caught me forum hopping!  You do it in the dead of the night!
 
I had a problem with the piston hitting the front cover, so I moved the cylinder about 1/32" forward and that stopped. I need to check the front and rear space is equal, but I think it will be. There is no piston nut and it simply a force screwed fit. The clunk occurs at front dead centre passing from forward to rearward movement. It may be a little play in the axle box in the horn.  However, the packing could be a point, I had to force it in and I suppose a PTFE ring would be a better bet.  Just that I would have to re make the piston to fit the ring and also make sure the PTFE working temperature was okay for SH steam.
 
By the way, with the boiler, thanks for Bryans link.  I forgot all about Don Young's 4F, that has a band at the throatplate.
 
All the best
 
 Dave

David Haynes28/07/2010 23:33:53
168 forum posts
26 photos
Just to bring things up to date, In order to locate the cyinder(s), the axleboxes were clamped at the height specified on the drawing but in doing so the boxes hadn't seated evenly causing the wheels to stiffen; slackened off, tried again and this improved things a little.  The piston is fairly free to move in the slide bar assembly and I believe all components are correct lengths.  The thing I have found awkward is how much stiffness is acceptable.  Whilst I don't expect any component to spin or slide freely under gravity or radial inertia when spun, neither do I expect anything to be locked solid.  It's knowing how much stiffness is okay! 80psi on a piston applies a lot of force, but I don't want to take this too much for granted.
 
The cylinder is true to distance from the driving wheel c/c and the piston rod is correctly aligned with the driving wheel c/c, the steam clearance at each end of the stroke seems to be about 1/32" and having drawn the layout to full size and sticking the drawing on the frame, I think it all seems the best I can do.
Any more suggestions are welcome!
Dave

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