BobCat S70 – CYLINDER (TILT) Testing

Remove the attachment. Tilt the Bob-Tach fully forward until it rests on the floor. Stop the engine. Raise the seat bar.

Disconnect the hose (Item 1) [Figure 20-21-1] from the base end of the tilt cylinder.
Install a plug in the hose and tighten the plug.
Lower the seat bar, engage the parking brake and start the engine.
Push the bottom (heel) if the tilt pedal until system relief pressure is reached.
There should be no leaks from the base end of the port.
If there is leakage from the open port of the tilt cylinder, remove the cylinder for repairs.

Remove the hoses (Item 1) [Figure 20-21-3] from the tilt cylinder fittings.

Remove the bolt (Item 1) and the retainer (Item 2) [Figure 20-21-4] from the base end of the tilt cylinder.

BobCat S70 – CYLINDER (LIFT) Disassembly And Assembly

Use the following tools to disassemble and assemble the cylinder:
MEL 1074 – O-ring Seal Hook
MEL 1396 – Seal Installtion Tool
MEL 1033 – Rod Seal Installation Tool
Piston Ring Compressor
Spanner Wrench
Hold the hydraulic cylinder over a drain pan and move the rod in and out slowly to remove the fluid from the cylinder.

Put the base end of the cylinder in a vise.

Use a spanner wrench to loosen the head [Figure 20-20­11].

Remove the head and the rod assembly from the cylinder [Figure 20-20-12].
Put the rod end in a vise.
Remove the nut (Item 1), piston (Item 2), spacer (Item 3) and head (Item 4) [Figure 20-20-12].
Assembly: Tighten the nut (Item 1) [Figure 20-20-12] to R300 ft.-lb. (407 N*m) torque.

Remove the seal (Item 1) and O-ring (Item 2) from the piston (Item 3) [Figure 20-20-13].

Remove the thick O-ring (Item 1) and the back-up washer (Item 2) from the groove in the head. Remove the thin O-ring (Item 3) [Figure 20-20-14].

Remove the wiper seal (Item 1), and rod seal (Item 2) [Figure 20-20-15].
Remove the O-ring (Item 3) [Figure 20-20-15] from the rod seal.
Wash the cylinder parts in solvent and air dry them.
Inspect the cylinder for nicks, scratches or other damage.
Replace any damaged parts.
Always install new O-rings and seals during assembly.
Lubricate all O-rings and seals with hydraulic oil during installation.

Install a new seal on the tool and slowly stretch it until it fits the piston (Item 1) [Figure 20-20-16].
Allow the seal the stretch for 30 seconds before installing it on the piston.
Use a ring compressor to compress the seal to the correct size. Leave the piston in the compressor for about three minutes.

Install the rod seal on the rod seal tool [Figure 20-20-17].
NOTE: During installation the O-ring side of the seal must be toward the inside of the cylinder.
Rotate the handles to collapse the rod seal.

Install the rod seal in the head [Figure 20-20-18].
Install the wiper seal with the wiper toward the outside of the head.
Assemble the cylinder(s) in the reverse order.

BobCat S70 – CYLINDER (LIFT) Removal And Installation

Fully lower the lift arms.
Stop the engine. Pull up on the lift arm by-pass control and move the lift pedal to release the hydraulic pressure. Raise the seat bar. Engage the parking brake. Raise the operator cab.

Install a lifting strap around the center of the lift arms crossmember, fasten the strap to a chain hoist and the lift arms slightly as shown [Figure 20-20-4].
Remove the rod end retainer bolt (Item 1) [Figure 20-20­ 4] from the left cylinder.
Installation: Tighten the bolt to 18-20 ft.-lb. (24-27 N*m) torque.
Remove the retainer and the rod end pin [Figure 20-20­ 4].

Lower the left cylinder onto the fender [Figure 20-20-5].

Remove the bolt (Item 1) [Figure 20-20-6] to remove the cover and pivot pin from the right lift cylinder.
Installation: Tighten the bolt to 18-20 ft.-lb. (24-27 N*m) torque.
Remove the cover and pivot pin [Figure 20-20-6].

Lower the right lift cylinder onto the fender [Figure 20-20­7].

Open the rear door.
Remove the bolt (Item 1) [Figure 20-20-8] from the base end pin retainer (left side shown).
Installation: Tighten the bolt to 18-20 ft.-lb. (24-27 N*m) torque.

Use a slide hammer to remove the pivot pin (left side shown) [Figure 20-20-9].

Pull the cylinder forward to remove the hose (Item 1) from the base end and the hose (Item 2) [Figure 20-2010] from the rod end. Mark the hoses for correct reassembly.

Install the cylinder(s) in the reverse order.

BobCat S70 – CYLINDER (LIFT) Testing

Open the rear door.

Remove the bolt (Item 1) [Figure 20-20-1] from the base end pin retainer (left side shown).

Use a slide hammer to remove the pivot pin (left side shown) [Figure 20-20-2].

Pull the cylinder forward. Mark the hoses for correct reassembly.

Check only one cylinder at a time.

Disconnect the hose (Item 1) [Figure 20-20-3] from the lift cylinder base end port.
Install a plug in the hose and tighten the plug.
Lower the seat bar, engage the parking brake and start the engine. Loaders without seat sensors press the green PRESS TO OPERATE button.
Push the top (toe) of the lift pedal until system relief pressure is reached.
If there is leakage from the open base end port of the lift cylinder, remove the lift cylinder for repair.

Spring-loaded-type hydraulic accumulators

A spring-loaded accumulator is similar to the weight-loaded type except that the piston is preloaded with a spring. A typical cross-section of this type of accumulator has been illustrated in Figure 7.15.

spring-loaded-accumulator

The spring is a source of energy, acting against the piston and forcing the fluid into the hydrauhc system. The pressure generated by this accumulator depends on the size and preloading of the spring. In addition, the pressure exerted on the fluid is not constant. They typically deliver small volumes of oil at low pressures and therefore tend to be heavy and large for high-pressure, large volume systems.

A spring-loaded accumulator should not be used for applications requiring high cycle rates as the spring may lose its elasticity and render the accumulator useless.

Hydraulic heat exchangers

Heat is generated in a hydraulic system because of the simple reason that no component can operate at 100% efficiency. Significant sources of heat include pumps, pressure relief valves and flow control valves. This can cause a rise in temperature of the hydraulic fluid above the normal operating range. Heat is continuously generated whenever the fluid flows from a high-pressure region to a low-pressure region, without producing mechanical work. Excessive temperatures hasten oxidation of the hydraulic fluid and also reduce its viscosity. This promotes deterioration of seals and packings and accelerates wear and tear of hydraulic components such as valves, pumps and actuators. This is the reason why temperature control is a must in hydraulic systems.

The steady-state temperature of the fluid depends on the rate of heat generation and the rate of heat dissipation. If the fluid-operating temperature is excessive, it means that the rate of heat dissipation is inadequate for the system. Assuming that the system is reasonably efficient, the solution is to increase the rate of heat dissipation. This is accomplished by the use of ‘coolers’, which are commonly known as heat exchangers. In certain applications, the fluid needs to be heated in order to achieve the required viscosity of the fluid in the system. For example, if a mobile hydraulic equipment is required to operate in sub-zero conditions, the fluid needs to be heated. In such cases, heat exchangers are termed as heaters.

The factors to be considered when sizing a heat exchanger are:
• The required drop in temperature of the hydraulic fluid
• The flow of the hydraulic fluid in the system
• The time required to cool the fluid.

There are two main types of heat dissipation heat exchangers:
1. Air-cooled heat exchangers and
2. Water-cooled heat exchangers.

Water-cooled hydraulic heat exchanger

Figure 7.26 is an illustration of a common type of water-cooled heat exchanger used in hydraulic systems.

This is typically a shell and tube-type heat exchanger. The cooling water is pumped into the heat exchanger and flows around the tube bank. The hydraulic fluid, which is to be cooled, flows through the tubes. While flowing through the tubes, the fluid gives away heat to the water, thereby reducing its temperature.

Advantages of water-cooled heat exchangers are:
1. They are very compact and cost-effective
2. They do not make noise
3. They are good in dirty environments.

Disadvantages associated with water-cooled heat exchangers are:
1. Water costs can be expensive
2. Possibility of mixing of oil and water in the event of rupture
3. Necessity for regular maintenance to clear mineral deposits.

water-cooled-hx

Gas-loaded-type hydraulic accumulators

These types of accumulators (frequently referred to as hydro-pneumatic accumulators) have been found to be more practically viable as compared with the weight and springloaded types. The gas-loaded type operates in accordance with Boyle’s law of gases, according to which the pressure of a gas is found to vary inversely with its volume for a constant temperature process.

The compressibility of the gas accounts for the storage of potential energy in these accumulators. This energy forces the oil out of the accumulator when the gas expands, due to a reduction in system pressure.

Gas-loaded accumulators fall under two main categories:
1. Non-separator type
2. Separator type.

Components of hydraulic systems

Virtually, all-hydraulic circuits are essentially the same regardless of the application. There are six basic components required for setting up a hydraulic system:

1. A reservoir to hold the liquid (usually hydraulic oil)
2. A pump to force the liquid through the system
3. An electric motor or other power source to drive the pump
4. Valves to control the liquid direction, pressure and flow rate
5. An actuator to convert the energy of the liquid into mechanical force or torque,
to do useful work. Actuators can either be cylinders which provide linear
motion or motors which provide rotary motion and
6. Piping to convey the liquid from one location to another.

Figure 9.1 illustrates the essential features of a basic hydraulic system with a linear hydraulic actuator.

The extent of sophistication and complexity of hydraulic systems vary depending on the specific application.

Each unit is a complete packaged power system containing its own electric motor, pump, shaft coupling, reservoir and miscellaneous piping, pressure gages, valves and other components required for operation.

hydraulic-system