Parallel Motion

[WolfieParticipant @wolfie]

[WolfieParticipant @wolfie]

OK well I think I'm probably being a bit thick here but after haviing read the ME Steam Engine special I still don't quite understand parallel motion.

I know that the theory is that to both pull and push a beam, the piston rod has to be attached to it directly. Now as a beam end describes an arc, that would mean the piston rod would be trying to describe an arc at the top end which is obviously not desirable. But why all those rods?

What was wrong with simply jointing the piston rod so that the top half pivoted and moved with the beam and the bottom half stayed straight??

[John McNamaraParticipant @johnmcnamara74883]

Hi Wolfie

This link may help explain the motion.

**LINK**

As it happens I just read about it in an old copy of Franz Reuleaux's book (English translation) as mentioned in the references at the bottom of the Wiki.

Another straight line motion Peaucellier-Lipkin

http://en.wikipedia.org/wiki/Peaucellier%E2%80%93Lipkin_linkage

Cheers

John

Edited By John McNamara on 28/06/2012 14:07:32

[Diane CarneyModerator @dianecarney30678]

Posted by Wolfie on 28/06/2012 13:12:50:

But why all those rods?

What was wrong with simply jointing the piston rod so that the top half pivoted and moved with the beam and the bottom half stayed straight??

Gings… why didn'ay I think o' that?

JW

[Martin WParticipant @martinw]

Wolfie

The reasoning behind it is to relieve any side strain on the end of the piston rod by, as it says, holding it parallel to the cylinder. If it wasn't there then the end of the piston rod would have a side thrust applied and the piston rod would act as a lever against the cylinder and piston. This would at the very best cause excessive wear and at worst could bend the piston rod or break the gland which it passes through. You will find various supports on most engines ranging from cross heads on steam locos to slides on mill engines and a range of alternatives between, I think your engine has a cylindrical guide and cross head to support the piston rod.

The parallel motion is just another way of distributing forces and keeping things in line, looks pretty neat when running as well.

Hope this ramble helps a bit

Cheers

Martin W

[WolfieParticipant @wolfie]

Posted by Martin W on 28/06/2012 15:39:58:

The reasoning behind it is to relieve any side strain on the end of the piston rod by, as it says, holding it parallel to the cylinder. If it wasn't there then the end of the piston rod would have a side thrust applied and the piston rod would act as a lever against the cylinder and piston. This would at the very best cause excessive wear and at worst could bend the piston rod or break the gland which it passes through. You will find various supports on most engines ranging from cross heads on steam locos to slides on mill engines and a range of alternatives between, I think your engine has a cylindrical guide and cross head to support the piston rod.

The parallel motion is just another way of distributing forces and keeping things in line, looks pretty neat when running as well.

Thats a nice little explanation, makes sense. Since I posted that question I have found an excellent little pdf that explains it thoroughly. I'll post the link later on

[LambtonParticipant @lambton]

"What was wrong with simply jointing the piston rod so that the top half pivoted and moved with the beam and the bottom half stayed straight??"

The piston rod would still be uncontrolled. The simpler way to arrange things by using a cross head for guidance was quickly introduced when the steam engine was adapted to power locomotives.

[Joseph RamonParticipant @josephramon28170]

I have never understood why it took so long for the crosshead guide to be invented – it seems so obvious and makes the whole construction of an engine so mucjh simpler. Presumably it is because of the evolutionary progress from atmospheric engines, but it is ironic that, aside from the valve gear, later engines were much simpler than earlier ones.

Another example is the time taken to use horizontal cylinders, although with the poor facilities for making truly circular cylinders in the very early days they would have struggled.

Joey

[LambtonParticipant @lambton]

As I understand it there was great reluctance in the early days to use horizontal cylinders as it was (not unreasonably at the time) believed the weight of the piston would cause excessive and premature wear along the bottom of the cylinder. When horizontal cylinders were eventually tried it became apparent that no abnormal wear actually took place and the rest is history.

[Jeff DaymanParticipant @jeffdayman43397]

In large scale vertical cylinder engines a crosshead and vertical guides for it may have been a major challenge to make and erect in place in the very early days. No lasers or maybe even machinist's levels in the early 1800's. A plumb bob would likely be the most accurate aligning tool at the time. The links in parallel motion are similar to other shop made engine parts and could be erected easily when building up the engine. Links are essentially just two accurate holes or openings at the ends of a bar, machinable in one setup on a drilling/boring machine. Crosshead guides are a different story. JD

Edited By Jeff Dayman on 29/06/2012 16:19:52

[KWILParticipant @kwil]

Early locos such as Locomotion had their own form of parallel motion, the lever arms and rocking motion produces quite an accurate straight line operation of the piston rod from the cross beams

[Sub MandrelParticipant @submandrel]

@Jeff – I think that, in practice, setting up parallel motion would be as skilled as setting up a guide. It's clear the manufacturers put great store in supplying their own engineers to supervise erection or at the least complained loudly of the incompetence of local engineers.

@Kwil – As far as I can see from smudgy reproductions of engravings, the Locomotion arrangement was very like the half-beam or 'grasshopper' arrangement.

Trevethick's slightly earlier Pen-y-Darren loco hade horizontal cylinder, a crosshead and guide rods – so some things were obviously going backwards!

Neil

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