Gripping force

[arcka]

Hello Guys,

I need help in calculating the gripping force. I have to use a pneumatic cylinder which has linear mition and I am planning to use scissor tongs to carry this motion into gripping force. Can anybody help me with the selection of cylinder w.r.t displacement or force?

Thanks in advance
Arcka

 

[dhengr]

Arcka:
You have to be far more descriptive than you have been in your OP. We can’t see it from here, nor do we have any idea what you are really trying to do. Do a simple, well proportioned sketch, with some dimensions, forced needed, general idea of the mechanism you are talking about, so we can see what you are thinking. There are plenty of smart people here willing to help you once we understand what you need and can see how to manipulate the variables.

 

[gerhardl]

A scissor fork is in principle a simple weighted lever (force times arm) where your cylinder will give the force. The force given to the gripping area of the scissor has to be strong enough to hold the weight secure, but not too high, crushing or damaging the item gripped.

A 'reasonable' sized cylinder will often deliver far more power than necessary. Ask a supplier, or buy a sylinder giving a total force considerably higher than the weight of the item. Experiment and limit the pressure to give needed force. pressure regulator, pressure indicator, pressure switch,throtteling valve,checkvalve when necessary, and correct sized items of all. Or in addition: downsize or enlarge the cylinder if necesarry.

You can also experiment and calculate to find the correct friction coefficient between the two surfaces and guesstimate necessary force from that.

Google physics for the formula.

 

[arcka]

Sorry here you can find the sketch of my idea. I have a constraint that I cannot use large cylinders because of the space and weight limitation. I need to aplify the force or in other words I need to get the higher force ratio.

 

[arcka]

Here you go.!

 

[desertfox]

Still no dimensions on the sketch, how are we supposed to calculate anything without dimensions?

 

[arcka]

Hi, I have not decided the dimensions because I am open with selection of dimension. This also part of my design along with the selection of cylinder. I am considering the length of the arms as unknown because I would not like to limit my options. I am going to adjust dimensions of the tongs as per the cylinder. Hope I am expressing my ideas clearly..

 

[gerhardl]

You said 'scissor', why instead drawn as an open claw?

 

[arcka]

The reason is I have found formulae for Scissor grip, and it was said that we cannot get high force amplification with scissor system. So I thought of open claw may work, but I am not sure about it either. I am unable to choose which is beter, and here I am asking for help ..!!

 

[desertfox]

Sadly without dimensions how do you know you cannot get enough force fro a scissor grip.
Normal practice would be to start with your load, workout the mechanism and then select a suitable cylinder, what you appear to be doing is the reverse.

 

[arcka]

@desertfox, for the sake of calculation, let us have the dimension (in the image). Can you tell me how can I calculate the cylinder force (F) now? Help would be appreicated.

 

[desertfox]

Hi

Well get rid of the two links dimensioned 90 and mount the cylinder horizontal between the top end of the links dimensioned 200.
Move the pivot points down as close to the grips marked 30, let's say the pivot points are 50 off the bottom end of the links where the grips are, now you have a ratio of 150/50 which is roughly 3.1 which means if you 25 iN force total at the gripper end you would need 8.3kN at the cylinder.

 

[rb1957]

the two arms of the claw are three force members ... a force along the link (that joins to F), a force through the pivot (how does that work ? you want to pick up this thing at only one location ?), and the force applied to the body. so you can draw the force polygon.

another day in paradise, or is paradise one day closer ?

 

[3DDave]

What problem are you having in understanding?
Need help with Geometry?
Need help with Trigonometry?
Need help with Conservation of energy?

 

[LittleInch]

The only advantage in your sketch is whether the actuator and force F is fixed in that orientation, i.e. you can't turn it 90 degrees as desert fox says.. Then force F is then factored by the sine of the internal angle/2 so long as it all stays symmetrical. However you can never get greater than 1 and often much less so your amplification form the lever effect needs to be even higher to get the same effect. Hence gripping force is a function of distance that the central pivot moves relative to the ends of the arms.

Pretty simple trigonometry.

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

 

[arcka]

Thanks guys, it was pretty informative. Forget my drawing. Is there a way method or mechanism or method to get maximum force ratio or force amplification? I have restriction of 25 KN of the load at my workpiece. Any hint will be appreicated. Need not be the method I sketched. Thanks inadvance

 

[rb1957]

you've heard of levers, right ?

as desertfox posted, if you have the hydraulic cyclinder driving both arms of the claw, and you connect the two hinge points together, then your 25kN force is applied with an 8kN cylinder. but stroke on the cylinder will be 3x the movement at the jaws.

another day in paradise, or is paradise one day closer ?

 

[desertfox]

Hi rb1957

Well I am pleased that you at least read and understood my post.

Thanks

Desertfox

 

[arcka]

Hello again,

I understand now that I do not get to choose the mechanical system, because this system already exists. My company needs to work on the alterations (changing only the cylinders - image attached). I would need to calculate Force 'F' at the end of the device. I will be thankful if someone helps me.

http://imgur.com/6aV5Fxk

 

[rb1957]

isn't it "simply" ...
you know the force from the hydraulic cylinder, applied to two links.
the links are 2 force members, reacting the applied hydraulic cylinder force axially
this axial force is causing torque on the gripper arm (about the white bolt)
which is applying force to the subject, P = Plink*190.6*sin(theta)/75

another day in paradise, or is paradise one day closer ?