# Mechanical advantage pickup truck crane

1. May 8, 2010

### m iket

I bought this truck crane for my dad. I want to calculate the mechanical advantage of it.

http://www.harborfreight.com/1-2-half-ton-capacity-pickup-truck-crane-1647.html [Broken]

I know how to calculate the mechanical advantage of a hydraulic jack and of a lever but im not sure about where they connect to each other. When the jack lengthens it rotates back so the different angles change. How do i calculate that? Where do i start. Does it involve the law of cosines and calculus or something simpler?

Last edited by a moderator: May 4, 2017
2. May 9, 2010

### m iket

Is this the right forum?

3. May 9, 2010

Hi there m_iket and welcome The Mechanical Engineering forum might be more appropriate, but I think that there might be some confusion regarding what exactly you are asking. Maybe it is a term that I am just not familiar with, but I am not sure what you mean by "mechanical advantage."

Can you state a little more precisely what you wish to calculate?

4. May 9, 2010

### m iket

I learned about that in middle school physics. I think i will ask in the mech engineering forum, im thinking about doing some modifiications to the the crane anyways

edit: I know how to calculate that for the boom, ratio of the distances to the fulcrum and for the hydraulic jack ratio of the piston diameters. However where they connect there's changing nonperpendicular angles and linear and rotation motion so im not sure how it works.

Last edited: May 9, 2010
5. May 10, 2010

### Staff: Mentor

Do you mean you're trying to find the total mechanical advantage of the lever arm of the jack vs the arm of the boom? I'm not sure that's something you can calculate without knowing the properties of the jack pump and it may not even be constant if it depends on the weight of what you are lifting (I assume it is a pneumatic cylinder?).

Also, in terms of force, it varies with the angle of the boom.

The best you might be ablel to do is to start with the boom horizontal and pick up an object of known weight. Pump the handle of the jack a few times and measure how far the boom moved, then compare to the distance the end of the handle moves.

6. May 10, 2010

### Mu naught

You'd have to know a lot to calculate it even roughly, like the PSI of the hydraulic system, accurate measurements of the angles and lengths of the arms etc...

i think it'd just be easier to take something you know is heavy, and attach it to the crane and see how much force it takes to life it up. That will give you a ratio which should be equivalent to what you want to know.

7. May 10, 2010

Well then, I learned two new things today! 1) Definition of MA (I would have understood it to be efficiency) and 2) they offer physics to middle-schoolers! I wish I had physics in middle school!

8. May 10, 2010

### m iket

Er i didnt have a physics only course in middle school, i had science classes but they covered basic physics(including simple machines) along with other sciences. I had my first physics course in high school.

I bought the thing, I can easily take measurements of the exact different lengths. I bet relative lengths could be determined with only a picture though. I was more looking for the forumla to use than the answer anyways. I dont think i would need to know the psi exactly, just the two different piston diameters. I though about putting a weight on the boom but wasnt sure how i would measure force on the handle, well i could buy a spring scale of some sort i guess It didnt occur to me till now i can put a smaller weights on the handle till it is more or less than the bigger weight on the boom.

I wanted to do the calculations though for the fun of it!!! Unfortunately i dont know how. I havent got the crane mounted to the truck yet, i need a friend to do some welding because where i want to mount it on the bed theres no frame members directly underneath to bolt down through to. It takes a large number of pumps to raise the boom, probably an excessive number for the loads im looking at . I thought about maybe modifying it to get the attachment point of the jack closer to the fulcrum so i would need less pumps. I also though about then moving the fulcrum pin up to increase the range of motion. It more likely i wont do that. I would just like to be able to do the calculations and have an understanding of how the angles and everything work.

I posted this here because i though it would be simple and the engineering subforum was for more complicated stuff. Also their terminology maybe different. If i do modify it then i definitly think that should be the place to ask.