Hydraulics questions for a project of mine...

[Noah Hepworth]

I want to start put by saying feel free to remove this post if this is not the appropriate forum. I am by no means and trained engineer, I just enjoy building/fabricating things myself instead of purchasing them when possible.

The current project I have in mind is essentially a wheel lift. The hydraulic "arm" that a tow truck has to lift and end of your vehicle. I am planning to build one of those that is a mostly isolated unit. I want to be able to transfer it between vehicles with the least amount of work required. I have a good vision of how I will physically build the device and where I want to place the hydraulics. That being said, I do not know very much about hydraulics and which ones to select so I have a few questions. I plan to over build the device with the aim that the arm will be able to lift up to 18,000 pounds. Question I have:

1. I have not firmed up the dimensions of my design but I am planning on using between an 8-10inch travel 4inch bore ram in a third order lever setup to improve travel distance of the load and speed of lifting. The ram I looked at today at a local store was rate for 35,000 pounds at 3000 psi. Once I firm up dimensions I believe this setup would actually be capable of around 20,000 pounds. So, will having a stronger ram than I need cause a noticeable impact on operation speed?

2. I don't need the lift to be lighting speed, but no slouch either. What is the best option for the hydraulic power unit? I am pretty set on staying with something diesel or electrically powered. Does my hydraulic power unit need to be capable of a greater psi than the max of the ram? As well, the vehicle the device is to be mounted on will have on demand supply of compressed air, could any air over hydraulic system ever handle a hydraulic ram this big?

Generally, I have fabricated tools and pieces of equipment that do not themselves move or do anything so this is a first for me. Any help or feedback would be appreciated! Please feel free to email me at noah.hepworth@hotmail.com.

 

[hydtools]

Your third order lever will require your cylinder to provide a force greater than your 18,000 pound load. By how much depends on the location of hinge, cylinder and load points. By ratio of travel,if piston moves 1 inch and load moves 2 inches the piston must provide force 2x the load. So how much piston force do you need? Can you share a sketch?

Ted

 

[Noah Hepworth]

I can share a sketch when I get home tomorrow. I wanted to use a third order lever as means of a distance magnifier. So I can use for ex a 10 in stroke cylinder and have the arm lift approx 18 in up. (Those are the rough numbers I have in my head pending what strength of hydraulics are within budget and a few measurements when I get home.

 

[zeusfaber]

For a given bore of ram, the strength rating won't affect the speed of operation - all it does is tell you how far you can load it without risk of bending something. The thing that governs the speed of operation is the volume of fluid that the power pack puts out.

The HPU doesn't need to be capable of providing more pressure than the ram can take without bursting - what you need to do is make sure that you understand the pressure that will be required to lift the maximum load you want to lift, then fit relief valves that will lift at that pressure plus a sensible margin (typically 10%), then make sure that all your components can handle that relief pressure plus another sensible margin.

Have you calculated what will happen to the axle loads on your towing vehicle? Hanging something heavy some way behind the back of the vehicle is a very good way to overload a rear axle and an even better way to unload a front axle to the point that you run out of steering and brake efficiency - which gets quite entertaining when you've got something heavy behind trying to push you around.

A.

 

[BEMPE16524]

Preferably a sketch for me to understand.

R.Efendy