# Hydraulic Pressure Control Applications

1. A backhoe operator controls the speed of the cylinder, which extends the bucket by cracking open the DCV controlling flow of the cylinder. Describe the difference in the “feel” of the controls using the two different relief valves (a) and (b) shown in Fig. 3.31.

2. A 1.79-in3 displacement motor is turning at 804 rpm and delivering 340 lbf-in of torque. The operator notices that motor speed decreases as load increases and the torque demand is greater than 340 lbf-in. The operator collected the following data:

Use this data to deduce the shape of the relief valve curve. Plot this curve [flow across relief valve (in3/s) vs. pressure (psi)].

3. A fixed-displacement pump is delivering 3.5 GPM to a circuit such as the one shown in Fig. 3.21. The workpiece must be held by the clamp cylinder with a minimum force equivalent to 600 psi during the operation. It has been determined that a clamping force equivalent to 750 psi will damage the workpiece.

Pressure at the cap end of the extend cylinder was measured during extension. An idealized plot of this pressure is shown in Fig. 3.32. (No data are presented for the retraction part of the cycle.)

a. Will this circuit operate satisfactorily? If so, briefly explain how all functional requirements are met.
b. Calculate the hydraulic energy (hp-h) dissipated across the sequence valve during extension. Remember energy is power × time.

4. A peanut shelling plant uses a truck dump to unload truck-trailers loaded with peanuts. The truck is backed onto a platform, and the entire vehicle (tractor and trailer) is raised to pour the peanuts out through the back unloading gate. The hose on the cap end of one lift cylinder failed at the fitting, and this caused the entire load to be held by the other cylinder. The resulting high pressure caused the hose on this cylinder to burst, and a \$105,000 truck was destroyed.

You are assigned the task of redesigning the hydraulic circuit such that a similar accident could never happen again. Your circuit is shown in Fig. 3.33. A counterbalance valve is installed directly on the cap end port of the cylinder. There is no line (hose or hydraulic tubing) between the port and the valve.

For simplicity, there is only one cylinder shown. The actual design has two cylinders with a counterbalance valve on each one.

Given:

Cylinders                           8-in. bore
5-in. rod diameter
Maximum lift force      100 tons

a. Find the setting on the counterbalance valve (psi) that will ensure that the load can never fall. Valve setting should be 10% higher than the pressure produced by the expected maximum load.

b. What pressure must be developed at the rod end of the cylinder to lower the load?

5. The stroke of the cylinders in Problem 4 is 18 ft. The design states that the truck must be lowered in 10 min. The counterbalance valve is a cartridge mounted in an aluminum block known as a line body. Total mass of the counterbalance valve is 9.4 lb. The pressure setting of the valve (inlet pressure) is 2200 psi, and the pressure drop from the outlet to the reservoir is 50 psi. Use the following assumptions and calculate the temperature rise of the counterbalance valve during a retraction event.

a. The valve is mounted on the cylinder. 20% of the total heat generated is conducted to the cylinder.
b. 40% of the total heat generated is exchanged to the oil as it passes through the valve.
c. 30% of the total heat is convected or radiated to the surrounding atmosphere during the event.

Specific heat of aluminum is 0.214 Btu/lbm · F. 1 hp = 2547 Btu/h.
Hint: Remember that hydraulic power is converted to heat energy when there is a pressure drop and no mechanical work is delivered.