knuckle joint lever design

[kevinkirk]

Hi All
I need to redesign a current lever operated system to knuckle joints to suit another machine with less available space. The current system uses a 50mm diameter, 150mm stroke air cylinder (5 Bar) acting on a 100mm long lever to rotate a shaft. on the other end of the shaft is a 75mm diameter bearing that is offset from the shaft centre line by 20mm. When the cylinder operates it turns the shaft 90 degrees. This lifts an arm by the required 22mm at one end.
I would like to change the lever operating method to a knuckle joint lever system using 2x 125mm long links and an air cylinder at the centre pivot.
Could someone help me with the calculations to find what the maximum force the current system can apply and the equivalent size of air cylinder in the new knuckle joint system.
Sketch's here
Cheers
Kevin

 

[desertfox]

Hi kevinkurk

I think we need more information about the mechanism in terms of the force at the other end of the lever that acts against the cylinder.
Looking at your sketches I don't think the idea is very good because you now have very poor mechanical advantage compared with the original design, also in the original design it seems the cylinder has a very long stroke when compared to the lever movement, which suggests that the mechanism as lost motion built in. The new idea seems to have no lost motion built in so how will the cylinder be controlled to get the right movement.

 

[kevinkirk]

Hi Desertfox
Thanks for the reply. I am unsure of the actual force used which is why I asked for the systems to have the same maximum force. My thoughts on the original design
1 The air cylinder has a max stroke of 150mm but only 140mm is used
2 The air cylinder has a max force of 982 Newtons (5 bar. 50mm diameter)
3 The torque is 15.8 Newton meters 100mm - 20mm * 982N
4 I don't know how to calculate any forces generated by the cam effect of the offset bearing
5 There are mechanical stops to limit the lever travel to 90 degrees.
6 The lever goes 10 degrees over centre to lock.

I hope this helps
Regards
Kevin

 

[chicopee]

If anything, I would think about relocating your cylinder in the opposite direction as it appears more effective. You should also make cutouts from rigid paper and analyze the motions of the models because I think that first the rod connection at the cylinder needs to pivot, secondly the link connection to the ground (where the 140d is labeled) appears as if it is stretching per your diagram which it would not unless it is a rubber band; the member to the link may be OK, however it is hard to know unless you lay out the motions on paper, cutouts or ACAD. Once the motions are established then the forces can be analyze but you will need to know the inertias of all components including those of the attached external components, acceptable angular and linear accelerations; frictional forces must not be discounted.

 

[kevinkirk]

Unfortunately there isn't space for a 150mm stroke air cylinder hence the need to redesign the linkage. I have found this website

http://www.excelcalcs.com/repository/machines/miscellaneous/scissor-lift-mechanism.xls/

that shows the linkage and mechanical advantage etc. Looking at the graph there is basically no mechanical advantage. As the old linkage has an 4 to 1 mechanical advantage by my reckoning I would need an air cylinder that gives me 4 times the force eg 100mm diameter.
Is my logic correct or am I over simplifying

 

[desertfox]

Hi

I think your on the right lines but when you obtain a bigger cylinder for the force you need it will probably have an increased length too.
Also you are working on the annulus side of the cylinder so when you do the maximum force ie pressure x area you need to take the rod area off the bore area, so your force is even less.
I don't think your mechanism will work very well but you really haven't give us all the information you have, so we are limited in what we can tell you.
Look at your proposed linkage its starts off over centre, now as the cylinder operates the links get closer to the vertical and when they get to the vertical position the force in them will be zero, prior to that the links will drive the lever in the wrong direction until they go over centre.
Its really not a good idea but without more information we are stuck like you.

 

[kevinkirk]

Hi
In my sketches the left hand pictures show the mechanism in the "on" position and the right hand show the "off" position. the linkage goes past centre to lock into position so that any downward forces on the top lever will not straighten the linkage. This means that the linkage only moves the top arm and does not hold it in position. As I have stated earlier the system will also have mechanical stops that will limit the linkage travel.
If you can tell me what further info you need I wil be happy to add it
Cheers
Kevin

 

[desertfox]

Hi Kevin

The point I was trying to make was what happens to the top lever when the cylinder pushes the connecting links from one side of centre to the other.

Regarding information, just look at the information you have and have much you have posted on here, the difference is what we need.
As it stands we don't even know what this lever's function is do you?

 

[kevinkirk]

Hi Desertfox
I have attached a picture that shows the complete original design. The lever pivots around the indicated point in the sketches and in doing so alters the centres of the other 2 holes, this in turn tightens a brakeband. I am adapting this brake system to another machine that is similar in design except for the main framework, and the only available position for the air cylinder mechanism is inside the framework.
I tried to keep my question simple regarding the linkage only, what it does is not important to the problem. basically I need to lift the left hand side of the lever 22mm with a system capable of equalling the max available force from the old system. I cannot use an air cylinder directly connected to the lever as it needs to lock in position.
Cheers
Kevin

 

[imcjoek]

First suggestion that comes to mind: buy a pneumatic brake.

From experience I can say this: be careful of knuckle links. They *will* bind up, if you give them half a reason to do so.