Chapter 6. HYDROSTATIC TRANSMISSION

6.1 Hydrodynamics Vs. Hydrostatics

Hydraulic transmission is a power transmission using fluid as medium. Today, there are many thousands of pressure-operated machines and they are so distinct from earlier devices . They are used very broadly in industries and in our life . The name hydraulics was applied to systems using liquids. In modern terminology, hydraulics implies a circuit using mineral oil. Hydraulics are divided into two sciences - hydrodynamics and hydrostatics.

Hydrodynamics deals with the characteristics of a liquid in motion, especially when the liquid impacts on an object and releases a part of its energy to do some useful work.

Hydrostatics deals with the potential energy available when a liquid is confined and pressurized. This potential energy also known as hydrostatic energy is applied in most of the hydraulic systems. This field of hydraulics is governed by Pascal's law.

It can thus be concluded that pressure energy is converted into mechanical motion in a hydrostatic device whereas kinetic energy is converted into mechanical energy in a hydrodynamic device.

Hydrodynamics can be called the science of moving liquids under pressure (We have studied already). A water wheel or turbine (Shown in the Figure) represents a hydrodynamic device. Energy is transmitted by the impact of a moving fluid against vanes. We are using the kinetic energy that the liquid contains.

A hydrodynamic transmission consists of non-positive displacement pumps and motors, such as turbine and centrifugal pumps the flow is continuous from inlet to outlet and results from energy being directly imparted from the fluid stream. These machines are basically low pressure with high volume output.

Transmission of energy using kinetic energy

The underlying principle of how fluids transmit power is revealed by Pascal's law. Pascal's law states "Pressure in a confined fluid is transmitted undiminished in every direction, and acts with equal force on equal areas, and at right angles to the container walls".

The transmitted pressure acts with equal force on every unit area of the containing vessel and in a direction at right angle to the surface of the vessel exposed to the liquid.

Pascal's law can be illustrated by the following example.

In a hydrostatic device, power is transmitted by pushing on a confined liquid (the below figure). The liquid must move or flow to cause motion, but the movement is incidental to the force output. A transfer of energy takes place because a quantity of liquid is subject to pressure. Most of the hydraulic machines in use today operate hydrostatically - that is through pressure.

6.2 Hydrostatic transmission - General conception

This type of transmission bases on uncompressible property of fluids to transfer energy, other words, fluid power is the use of a confined fluid flowing under pressure to transmit power from one location to another. The medium of transmission is a flowing fluid. Other words, HST is transmission of power by fluid using positive displacement

pumps and motors.

A hydrostatic transmission consists of a fixed or variable-displacement pump and a fixed or variable displacement motor, operating together in a circuit.

The primary function of any hydrostatic transmission (HST) is to accept rotary power from a prime mover (usually an electric motor or an internal combustion engine) having specific operating characteristics and transmit that energy to a load having its own operating characteristics. In the process, the HST generally must regulate speed, torque, power, or, in some cases, direction of rotation. Depending on its configuration, the HST can drive a load from full speed in one direction to full speed in the opposite direction, with infinite variation of speed between the two maximums - all with the prime mover operating at constant speed.

The operating principle of HSTs is simple: a pump, connected to the prime mover, generates flow to drive a hydraulic motor, which is connected to the load. If the displacement of the pump and motor are fixed, the HST simply acts as a gearbox to transmit power from the prime mover to the load. The overwhelming majority of HSTs, however, use a variable-displacement pump, motor, or both - so that speed, torque, or power can be regulated.

HSTs offer many important advantages over other forms of power transmission. Depending on its configuration, an HST:

- transmits high power in a compact size

- exhibits low inertia

- operates efficiently over a wide range of torque-to-speed ratios

- maintains controlled speed (even in reverse) regardless of load, within design limits

- maintains a preset speed accurately against driving or braking loads

- can transmit power from a single prime mover to multiple locations, even if position and orientation of the locations changes

- can remain stalled and undamaged under full load at low power loss

- does not creep at zero speed

- provides faster response than mechanical or electromechanical transmissions of comparable rating, and

- High sensation and accuracy in adjusting

- High stability in motion of actuator

- Smooth control and safety

- Ease and accuracy of control: By the use of simple levers and push buttons,the operator of a hydraulic system can easily start, stop, speed up and slow down.

- Multiplication of force: A fluid power system (without using cumbersome gears, pulleys and levers) can multiply forces simply and efficiently from a

fraction of a kilogram, to several hundred tons of output.

- Constant force and torque: Only fluid power systems are capable of providing a constant torque or force regardless of speed changes.

- Simple, safe and economical: In general, hydraulic systems use fewer moving parts in comparison with mechanical and electrical systems. Thus they

become simpler and easier to maintain.

Disadvantages

- Because of working with High pressure, it is difficult to seal

- Require high quality of oil

- Handling of hydraulic oils which can be quite messy. It is also very difficult to completely eliminate leakage in a hydraulic system.

- Hydraulic lines can burst causing serious human injuries.

- Most hydraulic fluids have a tendency to catch fire in the event of leakage,

especially in hot regions.

In the hydrostatic transmission there are three main components:

- Hydraulic pump

- Hydraulic motor (cylinder)

- Controlling and adjusting components (various valves)

A pump is required to push the fluid. An actuator is the output of the system. Besides a

pump and actuator, the

system also requires valves to control the fluid flow; a reservoir to store the fluid and supply it to the pump; connecting lines; and various hydraulic accessories.

Hydraulic Jack

A power-driven pump operates a rotary motor

Basic hydraulic transmission scheme

6.3 Operating principle of hydraulic transmission

6.3.1 Reciprocating motion

Hydraulic transmission with reciprocating motion

Left Figure:

1.Pump; 2. Discharge valve; 3.Suction valve; 4.reservor; 5.directional control valve; 6.actuator (hydraulic cylinder)

Right figure:

1. Pump; 2.safety valve; 3.directional control valve; 4.actuator (hydraulic cylinder)

6.3.2 Rotary motion

6.4 Hydraulic circuit types

6.4.1 Close circuits:

In a closed circuit, fluid from the motor outlet flows directly to the pump inlet, without returning to the tank. Because the pump and motor leak internally, which allows fluid to escape from the loop and drain back to the tank, a fixed-displacement pump called a charge pump is used to ensure that the loop remains full of fluid during normal operation and pressurizes the loop. The charge pump is normally installed on the back of the transmission pump and has an output of at least 20% of the transmission pump's output.

Advantages

- Supplement oil

- Increases suction pressure and so, increasing the system pressure, therefore this system can provides high power

- Reverse easily direction of actuator when it has high load

Disadvantages

- High temperature of circulating oil

- High ability of oil leakage

- High cost because of auxiliary components

6.4.2 Open circuits

Advantages

- Supplement oil easier

- Oil is cooled in reservoir

Disadvantages

- Supply smaller power

6.4.3 Differential circuits

6.5. Main components in static hydraulic transmission

6.5.1 Hydraulic pump

The pump is driven by a prime mover which is usually an electric motor or a petrol or diesel engine. The energy input from the prime mover to the pump is converted into high-pressure energy in the fluid which is transmitted through pipes and in turn is converted into rotational energy by a motor or translational energy by a cylinder.

When a hydraulic pump operates, it performs two functions. First, its mechanical action (the mechanical energy of the prime mover- an internal combustion engine or electric motor is transmitted to the pump) creates a vacuum at the pump inlet which allows atmospheric pressure to force liquid from the reservoir into the inlet line to the pump. Second, its mechanical action delivers this liquid to the pump outlet and forces it into the hydraulic system. A pump produces liquid movement or flow: it does not generate pressure. It produces the flow necessary for the development of pressure which is a function of resistance to fluid flow in the system. For example, the pressure of the fluid at the pump outlet is zero for a pump not connected to a system (load). Further, for a pump delivering into a system, the pressure will rise only to the level necessary to overcome the resistance of the load.

Hydraulic pumps are classified by their design. In all cases a moving element in a fixed container displaces fluid from an inlet to an outlet port. The pressure at the inlet port is due to the head of fluid from the reservoir and at the outlet port by the resistance imposed by the work load on the hydraulic motor/cylinder plus the resistance due to friction in pipes, valves, etc.

Classification of pumps:

- Gear pump (external gear pump and internal gear pump)

- Radial and axial piston pump

- Vane pump

a. Gear pump

b. Axial, radial Piston pump

Radial piston pump

Swash plate type axial piston pump

Axial piston pump with the constant horse-power device

1. Vít điều chỉnh áp suất 9. Đĩa nghiêng

2. Van điều chỉnh tải 10. Đĩa chặn

3. Con trượt 11. Piston

4. Piston điều khiển 12. Lò xo

5. Lỗ dẫn dầu vào, ra 13. Xi lanh

6. Khớp bán cầu 14. Đĩa phân phối dầu

7. Ổ đỡ 15. Cửa đẩy

8. Trục 16. Cửa hút

c. Vane pump

d. Symbol of hydraulic pump

a b c d

6.5.2 Hydraulic actuators

The actuator is the system's output component. It converts pressure energy to mechanical energy. They are like the muscles of the arms and legs in the human body. Hydraulic cylinders and hydraulic motors are actuators.

A cylinder is a linear actuator. Its outputs are force and straight line motion. A motor is a rotary actuator. Its outputs are torque and rotating motion.

Hydraulic actuator is divided into two type:

- Hydraulic cylinder

- Hydraulic motor

a. Hydraulic cylinder:

The cylinder consists of a ram or piston-type actuating cylinder. This converts the energy pressure into straight line motion.

These are broken down into two types depending on what kind of motion they can perform. The two types are "single-acting cylinders" and "double-acting cylinders".

Double -acting cylinder

Single-acting cylinder

Ram-type cylinder

b. Hydraulic motor

Hydraulic motors are actuators that convert pressure energy into continuous rotational movement.

Hydraulic motors can be classified into two types:

1. Limited rotation hydraulic motors

2. Continuous rotation hydraulic motors

Continuous rotation hydraulic motors

Limited rotation hydraulic motors

c. Symbol of actuator

a b c d

e f

6.5.3 Controlling and regulating components

These components are various valves that are classified as follows:

 1. Directional control valves

 2. Pressure control valves, and

 3. Flow or volume control valves

Some valves, however, have multiple functions that fall into more than one of these categories.

6.5.3.1. Directional control valve

Distributing mechanism is used to change flow direction at joints of pipe net and to distribute fluid into pipes which follow certain rule.

Directional control valves belong to the group of valves controlling flow direction. Their purpose is to direct pump flow to an actuator as well as allow return flow from the same actuator to the reservoir. They are classified according to the number of service ports and number of possible configurations (positions).

Directional control valves are classified into three types:

- Sliding spool type (Direct and Pilot operated Directional control)

- Rotary type

- Poppet type

Sliding spool type

Rotary type

Poppet type

6.5.3.2. Non return valve (Check valve)

This is a value that allows oil to flow freely in one direction, but stop it from flowing in the opposite direction (Check valves belong to the group of valves controlling flow direction. They act as rectifiers in a hydraulic system, allowing (almost) free flow in one direction and preventing flow in the opposite direction.

There are two types of Check valves. They are direct operated and pilot operated check valve.

a. Direct operated check valve

b. Pilot operated check valve

6.5.3.3. Flow control Valves (throttle valve)

Flow control valves provide volume control in hydraulic circuits. Flow is controlled by either throttling or diverting the flow. Throttling the flow involves decreasing the size of an opening until all of the flow cannot pass through the orifice. Bypassing the flow involves routing part of the flow around the circuit so that the actuator device receives only the portion of flow needed to perform its task.

Hydraulic circuits that use flow control devices are called metered circuits. If an actuator has the inlet flow controlled, the circuits is a "meter-in" circuit. If an actuator has the outlet flow controlled, the circuit is a "meter-out" circuit. Apart from theses circuits, there is a by-pass (bleed-off) circuit. Flow control circuits can either be non-compensated or compensated circuits.

These valves are divided into two types:

a. Non-compensated flow control valve

Throttle valve is used to adjust or resist flow rate (to control volume of oil to the hydraulic actuator) in system by making resistance to flow.

Non-compensated flow controls are simple valves that meter flow by throttling. The amount of flow that passes through the valve is determined by the position of the valve. As the valve is closed, flow decreases. One of the most common non-compensated valves is the adjustable needle valve.

Non-compensated flow control valve with adjustable orifice

The flow through the valve is calculated by the following fomular:

Where:

b. Pressure compensated flow control valve

Its purpose is to provide a constant flow independent of downstream pressure, i.e., actuator load (to maintain a constant pressure drop across the restrictor) in order to keep the speed of actuator constant independent of actuator load.

6.5.3.4. Pressure control valves

A pressure control valve may have the job of limiting or otherwise regulating pressure or creating a particular pressure condition required for control.

All pure pressure control valves operate in a condition approaching hydraulic balance. Usually the balance is very simple: Pressure is effective on one side or end of a ball, poppet or spool - and is opposed by a spring. In operation, the valve takes a position where the hydraulic pressure exactly balances the spring force.

Pressure-control valves are found in virtually every hydraulic system, and they assist in a variety of functions.

6.5.3.4.1 Safety valve

This valve is used to ensure for system in safety when pressure in system reaches the preset pressure range (It keeps the system pressures safely below a desired upper limit). This valve is also termed a maximum-pressure valve or safety valve. It normally is installed on main line with high pressure.

If there is no escape route for the hydraulic oil while its pressure increases continuously the hydraulic circuit can burst. A relief valve that allows the oil in the circuit to escape back into the tank when the pressure exceeds a certain limit in order to prevent the kind of malfunction described above.

6.5.3.4.2. Relief valve

It maintains a set pressure in a system or in part of a circuit.

The structure of safety valve and relief valve are the same.

a. Direct operated safety valve and relief valve

b. Pilot operated safety valve and pressure relief valve.

6.5.3.4.3. Pressure reducing valve

A pressure reducing valve is used to limit pressure level from the normal operating

pressure of the primary hydraulic system to the required pressure of a secondary

hydraulic circuit.

The purpose of pressure reducing valves is to maintain a desired pressure downstream of the valve, independently of (but lower than) the upstream pressure.

Pressure-reducing valves provide a steady pressure into a system that operates at a lower pressure than the supply system. A reducing valve can normally be set for any desired downstream pressure within the design limits of the valve. Once the valve is set, the reduced pressure will be maintained regardless of changes in supply pressure (as long as the supply pressure is at least as high as the reduced pressure desired) and regardless of the system load, providing the load does not exceed the design capacity of the reducer.

There are various designs and types of pressure-reducing valves. They are spring-loaded reducing valve and the pilot-operated reducing valve.

a. Direct operated reducing valve

b. Pilot-operated Pressure reducing valve

6.5.3.4.4.Counterbalance valves (Brake valve;Lower limit valve)

Which control a load induced pressure to hold and control the motion of a load.. This group valves provides balancing forces which prevent the load from running away because of its own weight or because of inertia.

Counterbalance valve stops flow from its inlet port to its outlet port until pressure at the inlet port overcomes adjusting spring force. An integral check valve permits free flow through the valve in the opposite direction.

Counterbalance valve

System lowering a load by means of a counterbalance valve.

6.5.3.4.5. Sequence valves

Sequence valves are used to assure that one operation has been completed before another function is performed. They operate by isolating the secondary circuit from the primary circuit until the set pressure is achieved.

A sequence valve is placed in a hydraulic system to delay the operation of one portion of that system until another portion of the same system has functioned.

Sequence valve (structure and symbol)

Sequence valve with internal & external signal input

6.5.3.4.6. Unloading valves

These valves are normally used to unload pumps. They direct pump output flow (often the output of one of the pumps in a multi-pump system) directly to reservoir at low pressure, after system pressure has been reached. They are usually used in circuits with two or more pumps or in circuits incorporating accumulators.

Unloading valve for accumulator circuit opens at a set unloading pressure and closes at a lower pressure. The valve opens when the system reaches a pressure determined by the adjustable spring and pump pressure on the right of the control piston. The valve closes at a lower pressure because force from system pressure on the left of the control spool must only overcome force of the adjusting spring.

Unloading valve in accumulator circuit Symbol

Application of unloading valve

6.5. 3.4.7. Hydraulic Fuse.

A hydraulic fuse is analogous to an electric fuse and its application in a hydraulic system is much the same as that of an electric fuse in an electrical circuit. A hydraulic fuse when incorporated in a hydraulic system, prevents the hydraulic pressure from exceeding the allowable value in order to protect the circuit components from damage. When the hydraulic pressure exceeds the design value, the thin metal disk

ruptures, to relieve the pressure and the fluid is drained back to the tank. After rupture, a new metal disk needs to be inserted before the start of the operation.

6.5. 4. Various hydraulic accessories

a. Reservoir

The hydraulic reservoir is a container for holding the fluid required to supply the system, including a reserve to cover any losses from minor leakage and evaporation.

b.Filters

Contamination of hydraulic fluid is one of the common causes of hydraulic system troubles. Installing filter units in the pressure and return lines of a hydraulic system allows contamination to be removed from the fluid before it reaches the various operating components. Filters of this type are referred to as line filters.

Filter with built-in pressure differential indicator

c. Accumulator

Accumulators are devices, which simply store energy in the form of fluid under pressure. This energy is in the form of potential energy of an incompressible fluid, held under pressure by an external source against some dynamic force.

This dynamic force can come from three different sources: gravity, mechanical springs or compressed gases. The stored potential energy in the accumulator is the quick secondary source of fluid power capable of doing work as required by the system. This ability of the accumulators to store excess energy and release it when required, makes them useful tools for improving hydraulic efficiency, whenever needed.

Gas-loaded type Accumulator Weight-loaded accumulator

Spring-loaded accumulator

d. Coolers

e. Pressure gauge and pressure switch

f. Thermometer and thermostat

6.6. Methods of speed control for actuators

There are two main methods to regulate the speed of actuator:

 Regulating the speed of actuator by varying the specific displacement of pump or hydraulic motors. Variable displacement pumps or motors are applied.

 Regulating the speed of actuator by throttling the oil volume to hydraulic motors (meter-in, meter-out and bypass or bleed-off circuit).

In fact, both of methods are used

Meter in speed control of Meter out speed control of

hydraulic cylinder hydraulic cylinder

By-pass (bleed-off) speed control of a hydraulic cylinder

6.7. Hydraulic oil

The choice of the most suitable hydraulic oil is of decisive importance for the faultless functioning, operational reliability, long service life and profitability of a hydraulic system. Hydraulic oil includes Mineral oil, Synthetic oil and Bio-oil

Properties of hydraulic oil:

 Low compressible: The incompressibility property of the fluid due to which energy transfer takes place from the input side to the output side (Transfer of hydraulic power from the pump to the hydraulic motor or cylinders (actuator).

 Low temperature sensitivity (Higher thermal stability). Viscosity varies a little when temperature of the fluid changes.

 Compatibility with sealing materials

 The removal of contamination and dirt, abrasive, water air etc.

 Low internal friction

 Good anti-stick properties

 Good hydrolytic stability (water separation).

 Increased fire resistance

 Compliance with foodstuff regulations etc.

 Lubricity-Lubrication of moving parts

 Air-separating ability

 Resistance to oxidation

 Sealing of clearances between mating parts: The fluid between the piston and the wall acts as sealant.

 Protection of the metal surfaces actually wetted by the hydraulic oil (protection against corrosion).

 The removal of heat losses which have been caused by leakage and friction losses.

 High filterability.

 Foam resistance

 Non-toxicity

These characteristics of hydraulic oil depend on the particular operating conditions.

Question.

1. What is the hydrostatic transmission. Explain its advantages and disadvantages.

2. State the basic hydraulic circuits

3. Draw symbols of hydraulic elements in hydrostatic transmission and state their function.

4. Draw the basic hydrostatic transmission scheme (Pump, actuators, valves and piping etc.) Explain their function.

5. State methods used to regulate speed of the actuator in the hydrostatic transmission.

6. State properties of hydraulic oil.