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Tich cylinders

Measurement wanted

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Michael Brett 121/03/2021 12:14:49
23 forum posts

Hi all

I have acquired a pair of Tich 3.5 inch gauge cylinders that I intend to use as the basis for a twin cylinder horizontal mill engine. My question is does anybody know what the piston stroke is on this engine. I just need the distance between the centre of the big end crank to the centre of the small end of the crank.

Many thanks Mike

JasonB21/03/2021 13:20:54
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I would not worry two much about the Tich crank throw as you want a mill engine to have as long a stroke as possible. So work out the longest length you can get out of the castings so the flanges just clean up and then deduct the following.

2 x 1/32" for spigots on the cylinder end covers

2 x 0.025" for clearance between covers and piston

1 x 1/4" or less for the piston, you could probably get it down to 3/16" if you used a single Viton O ring

Nicholas Farr21/03/2021 13:38:22
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Hi Michael, I'd be inclined to follow what JasonB has said, but if you really want to know, the drawing states 1-3/4" for length of the cylinder and 1/16" on each projection of the end covers and the piston is 7/16"

Regards Nick.

Michael Brett 121/03/2021 19:16:58
23 forum posts

Many thanks for replies. I should have made it clear that the cylinders are complete and finished , so no chance to alter that. Just to get this right as I understand it. Cylinders are 1 3/4 inch minus 1/8 inch for caps = 1 5/8 inch minus about 1/2 for piston , so distance between crank centres would be 1 1/8 inch, correct ?

Mike

Michael Brett 121/03/2021 19:19:23
23 forum posts

No think I made a mistake there , distance between crank centres would be half that , 1/2 plus 1/16.

Mike

Nicholas Farr21/03/2021 19:38:39
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Hi Michael, 9/16" is what the crank pin centres are on the drawing.

Regards Nick.

JasonB21/03/2021 19:59:20
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Well even if length of cylinder is set you can still stretch it out a bit. Half the covers to 1/32" which is more than adequate on such a small bore as is a 5/16" piston and a bit of clearance and you could get it to 21/32" crank throw which equals 1 5/16" stroke, more is you thin the piston down a bit further. So you are close to stroke being double the bore which is common on slow running mill engines rather than faster running locos.

Michael Brett 126/03/2021 15:40:14
23 forum posts

HI again

I have ended up with a throw on the crankpin of 17/32 between centres. All I need to know now is what is the throw between centres on the eccentric.

Any ideas Mike

Nick Clarke 326/03/2021 17:47:34
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eccentric.jpg

Michael Brett 126/03/2021 20:30:26
23 forum posts

Hi Nick

Many thanks for the information.

Regards Mike

Michael Brett 105/04/2021 10:53:50
23 forum posts

From the diagram above I read the eccentric throw as 9/64 x 2 which is about 7.2 mm throw.

From another site on the web someone has stated that this does not give enough port opening and the throw should be 11.13 mm .

Now I am confused , which is correct.

Mike

duncan webster05/04/2021 12:58:30
3509 forum posts
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If you just change the throw without changing the lap you'll also change the cutoff. I've got Tich drawings scanned somewhere, they were a freeby pullout with ME at some time. If you pm me your email I'll see if I can find them

JasonB05/04/2021 13:14:58
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What are you intending to do with the mill engine? There is a lot of difference between a Tich being able to pull it's driver around a track on 50+psi of steam to having a mill engine ticking over on a few psi of air . If the Later then you won't be lacking opening.

Nicholas Farr05/04/2021 13:17:52
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Hi, Nick Clarke 3's diagram is what is printed in the original publication. I don't know if it is an error of LBSC's or a typo though.

Regards Nick.

Nick Clarke 305/04/2021 14:17:16
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I don't think the drawings are incorrect - if for no other reason than so many of the locos have been built from them.

Inlet ports are 3/32, exhaust 1/4 with 3/32 bars giving a width across the inlet ports of 5/8"

The valve is 3/4" long so in mid travel overlaps the ports by 1/16" each side.

With a 9/64 throw eccentric this means the inlet ports open 5/64 out of 3/32 which is fairly normal as ports on LBSC's designs don't always open completely. There is no reason to suppose the extra 2mm or so will be needed - it still will not open the posts completely.

Michael Brett 105/04/2021 16:11:45
23 forum posts

Many thanks for your replies . I have now taken the plunge and machined the eccentrics to the drawings kindly posted by Nick Clarke. Moving the valve seems to open the ports the same as other engines I have built so all seems as it should.

Mike

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