Calculate hydraulic cylinder and pump forces on backhoe

Question

I have a field find mini backhoe from a shut down manufacturer. The product is a beaver Pro QT50 backhoe used for small tractors, atvs, and pick up trucks. Unfortunately, there's no documentation on this due to its age (as best I can tell it has spent about 20 years in the field). Because it's a field find, I think it needs all new hydraulic hoses, cylinders, controls , fluid, and pump but have no idea what specs these are.

It's a mini backhoe with rotate, lift, bucket, inner arm, and outer arm hydraulic cylinders. Your arm and outer arm measure 4 ft long the rotate and lift hydraulic cylinders are small being about 1.2 in in piston diameter and about two in cylinder diameter. The other three hydraulic cylinders are 1.8 in piston diameter and about 2.8 in cylinder diameter.

The specifications I have been able to find so far are:

1. Max lift weight rating of 500-1800 lb, 1845 lb Breakaway Force. I think this means 500 lb when the arm is extended and 1800 when it's fully retracted.
2. 1.7 gpm hydraulic pump
3. 490 lbs total weight
4. 200 bar pressure release valve
5. 4hp engine

Unfortunately I don't have a pressure rating for the hydraulic pump or lift capacity of each of the cylinders. How can I calculate these two figures?

Even ballpark figures will help as I'm trying to figure out if I can use electric actuators instead of hydraulics here. If the cylinders only need to have say 2,000 lb of lifting Force then electrical actuators are fine, but if they need 20,000lbs then it would not be worth the cost to buy electric actuators. Even if using hydraulics here, it would be very helpful to know which cylinders to buy.

I've put the measurements in a photo below

Answer 1

The first thing that comes to mind is that hydraulic actuators are better suited for this type of operation, so I would think twice before changing this.

For the pressure rating you can get a ball park figure from :

$$Power = Pressure \times Q $$

where $Q$ is the flow rate (you already have it). So you can solve for Pressure and you should get a ballpark figure (in the same order of magnitude) to the 200 bar (if it was properly designed).

(I am a little rusty with Imperial units so I won't give you a numerical result, but I also add that if Power is in hp, Pressure in PsiG and Q in GPM then the formula is $$Power [HP]=\frac{Pressure[Psig]\times Q[GPM] }{1714}$$

Regarding the forces of the cylinders, you can calculate the force if you know the internal diameter (usually for extending the force is greater, but you can get a ball park figure). The formula is quite simple:

$$F = P\cdot A$$

where:

• P is the pressure (should be at most 200bar that the hoses are rated - usually the safety factor is 4, so you might have to worry only about 50bar ).
• A is the area of the cross-section of the cylinder. I.e. if the internal diameter is d then $A= \pi \frac{d^2}{4}$ you should be able to find a ball park figure of the internal diameter from other cylinder of the same diameter (again I am not familiar with the imperial units market so I can't really comment).

These should give you an understanding regarding the specs of the electrical motors.