Linkage Torque/calculation

[gmcgrory]

Hi guys, im just new to this forum as a member but have been reading it for advice for a few months now, some expert advice indeed.

I am basically looking for some guidance on how to calculate the forces and torques involved in a basic linkage, which is the linkage used to curl the bucket on a back actor of an excavator. I am using it to rotate a conveyor through 180degrees. As the linear actuator (in my case an hydraulic ram) retracts it produces a moment on the arced swinging link which causes the straight link to provide a moment arm on the central large pin providing me with the rotation i need.

i would assume my ram creates a moment on the arced swinging link of 1400mm x max ram force( 53KN). At this point i dont know how to continue with the calculation to determine the maximum torque available at my main pin.

could any one tell me what i need to do to solve this problem? i have attached a jpeg of the linkage with a few dimensions i think i need

many thanks for reading and i look forward to some discussion on this

 

[gmcgrory]

Any thoughts or suggestions at all welcome guys

 

[rb1957]

couldn't open your file.

how about free body diagrams ?

 

[gmcgrory]

here is a PDF document attached which i hope can be opened rb1957 (and others).

let me know if this works guys and i appreciate all replies or ideas

thanks

 

[rb1957]

oops ... no attachment

 

[gmcgrory]

there appears to be a problem with the upload file feature on this site. is there any other way i can paste in an image or anything to this text message area?

i tried to paste a JPEG but it doesnt appear to allow that.

 

[btrueblood]

" the linkage used to curl the bucket on a back actor of an excavator"

like this?:

http://www.google.com/imgres?imgurl...1&ndsp=20&ved=1t:429,r:4,s:0&biw=1419&bih=873

There is no moment carried in the curved links (obvious from the pinned connections at their ends), they simply support the ram end out and away from the dipper (outbard link of the boom).

 

[SnTMan]

gmcgrory, I think if you review your statics, you will find that the moment on the large pin at any position is the product of the force applied by the actuator and the perpendicular distance pin-to-actuator. Neglecting losses.

Regards,

Mike

 

[gmcgrory]

I think the PDF has now successfully attached guys.... let me know if it hasnt

 

[gmcgrory]

btrue blood, i am indeed talking about a similar linkage to the one you posted. but the one im looking at is slightly different. the curved link in mine is joined to the link on the main pin via a straight link.

what is the the dynamics of this situation? is this a difficult problem for me to ascertain what is the maximum torque i can get from my pin. is it just simply as u say the perpindicular distance from the line of force of my ram to the main pin, or do i have to take into account the other links aswell?

 

[rb1957]

having seen your pdf ... free body diagrams, two force members ... each link, for different positions ... lick o'paint

 

[gmcgrory]

rb1957, can u explain maybe a bit further or give me an idea how to do the calculation. I done this kind of stuff in my degree but since i left uni i have not done much with regards to linkage calculations in my current work.

thanks guys

 

[rb1957]

google "free body diagram", get out an old engineering mechanics text ... the force in each of your links is along a line of action between the two lug centers (at the ends of each link). the curved arm will have some bending in it. you'll need to rotate the geometry to understand how the loads change. where is the load being applied ?

 

[btrueblood]

Like Rb says, draw your free body diagrams. Go pin by pin, and draw the force vectors. The curved link still transmits no moment to its fixed pin, unless I'm mistaking your diagram. In your configuration, it is just acting to guide the motion of the actuator rod end. Replace it in your schematic drawing as a rigid, straight rod linking the two endpoints (use dotted lines if it helps).

 

[gmcgrory]

hi guys.. i have just read through a lengthy revision of free body diagrams and understanding it pretty well. but i am still struggling to figure out how to solve my problem.

im not sure if the diagram is clear or not. tomorrow i will post another pdf of the linkage with the ram fully closed (i.e. when it is at 800mm closed centres) to maybe illustrate better what i mean.

this link will in essence be trying to rotate a mass of 4tonne which will be pivotal around the main large pin.

so im trying to figure will ram force be sufficient to allow this linkage to apply a sufficient torque arm to do this.

 

[zekeman]

The key is to take the leftmost pivot where three links intersect and place it in equilibrium (i.e.the vector sum of the forces there is zero), with all 3 links having forces along their axes (the circular link is made kinematically straight as suggested).
From this you should get the torque delivered to the main shaft which is the the force in the almost vertical link x the crank arm to the main output shaft x the sine of the angle between the 2 arms. You get this by resolving the almost vertical link into 2 vectors, one along the crank link and one normal to it, the one along is the inline crank force and the other the force which part of the couple that makes the torque or F*sine @.

 

[SnTMan]

lengthy revision of free body diagrams

Dammm! Gotta read it all again I suppose. I hope they marked them...

Regards,

Mike

 

[gmcgrory]

Here is another PDF showing the linkage fully closed and the linkage fully open. hopefully giving a better understanding of the dynamics involved and how it operates.

i will try what you said zekeman and see can i do this.

btw the max retraction ram force is 53KN, the mass the main pin will be rotating is 4000Kg ( my conveyor)

thanks

 

[gmcgrory]

looking at this for some time now, but cant get my head around this, never was much good at the free body diagrams!

do the reaction forces at the top 2 fixed pivot points nt add a number of unknowns to our problem?

is 1 force and the link lengths etc enough to solve this?

 

[gmcgrory]

I have sketched up roughly what i think the FBD for the full system looks like. i have added the reactions at the fixed points b AND c, as im sure these are important to consider. ill assume the links are of negligible weight also.


could any one guide me from here.... im not sure how to resolve this as i think i have too many unknowns.

i have added a PDF of how i think the FBD would look.

all help greatly appreciated guys.