linkage for parallel motion needed

[picclock]

Hi knowledgeable experts.

I have two pushrods offset radially, which having pressure seals, need to be moved axially. One pushrod activates the other via the linkage. I have seen pictures of beam engines which use a linkage at one end of the beam where it connect to the cylinder, and am considering using a similar system with the linkage at both ends of the 'beam' to achieve the same result.
Ideally I would like to get some mechanical advantage from the linkage, such that one pushrod would move twice the distance of the other. Can anyone point me to design guidelines or formula used for getting the correct link geometry.

However, I am also wondering if there is a simpler way to achieve the same result with a different linkage design.

Any help much appreciated

Best Regards

picclock

 

[gbritnell]

Hi Picclock,
As for me I could use a little more information, (offset radially) from the same center? Moved axially, (they rotate)? A beam linkage only keeps the piston rod supported by using the parallel motion. It takes the place of a crosshead, more or less.
To gain more travel from a given source then you would need an intermediate link that would have it's distance from the pivot 1/2 of what the source center distance was. Automotive rocker arms have mechanical gain from the pushrod but not to the extent of which you're asking.
gbritnell

 

[flyingtractors1]

Hi Picclock,

I'm not sure that I understand your plight, but have a couple thoughts you may consider. Written language would only comlpicate the matter (that is - I'm not sure that I could express my thoughts verbally), so maybe a sketch will help. Ralph

 

[picclock]

My apologies for not explaining the issue more clearly. I have two push rods (no rotational motion) parallel, but offset. These need to be linked such that a movement (push) on one will result in a movement of the other. Ideally, I would like a ratio, such that a 2" movement on one would result in a 1" movement of the other. The tricky part is that neither shaft should be subject to side loads, such as would occur using a simple lever.

I found this site http://www.mekanizmalar.com/menu_linkage.html to be quite informative. I originally planned on using a parallelogram movement (Watt linkage) as seen on the left hand end of the beam at this site http://www.animatedengines.com/watt.html

Using a lever incorporating the two straight line parallelograms mechanisms at each end would do the job but with a fair degree of complexity. Consequently, I am trying to find a simpler way.

Many thanks.

Best Regards

picclock

 

[Blogwitch]

Picclock,

Going by your example, maybe not an easy way, look at the pic at the bottom.

The other way would be to use something like the end off a 'nodding donkey' on each end of a beam, again with an offset pivot, but it would need spring or weight assistance to keep the chain/rope tensioned.

John

 

[picclock]

Hi John

That's what I first had in mind, except that the bar would be longer and pivoted at one end so that the two rods move in phase. I know it will work, although I'm not to sure of the dimensions of the parallelogram to get the best straight line action. Do you have a link to the calculation of such mechanisms ?

I was hoping to find something a little simpler but its not looking too promising atm.

Many thanks for the sketch, at least that illustrates the problem clearly.

Best Regards

picclock

 

[Blogwitch]

To do it much simpler you could use a connecting rod, like from a piston to a crank and use a crosshead on each one. But I don't know how much of a power loss you would get and how much swing you could get out of the crosshead/conrod combination before it started to get unstable.

John

 

[raggle]

Daft suggestion #173 - rack and pinion

Hope it's not too daft. 2 tooth belt pulleys, one twice the tooth count of the other, pinned together on a common shaft. The racks are pieces of matching belt cemented to the pushrod ends.

Push the rod on the large pulley and the other rod moves half the distance. Direction depends on which side of the pulley is driving. Sorry, no sketch - don't know how yet.

You will be stuck with a range of centres apart for the rods because of your choice of pulley diameters, and a back up roller may be needed to keep the rack engaged and not bend the rod.

I am highly skilled at seeing sticks and grabbing them by the wrong end, so this may well be rubbish.

Ray