Hydraulic Power Unit

Hydraulic Reservoir

Hydraulic Reservoir

Functions-1. To provide a chamber in which any change in the volume

of fluid in a hydraulic circuit can be accommodated.

2. To serve as a storage space for the hydraulic fluid used

in the system.

3. It is used as the location where the fluid is conditioned.

4. To provide a volume of fluid which is relatively stationery

to allow entrained air to separate out and heavy

contaminants to settle. The reservoir is where sludge,

water and metal slips settle.

5. It is a place where the entrained air picked up by the oil

is allowed to escape.

6. To accomplish the dissipation of heat by its proper design

and to provide a radiating and convective surface to

allow the fluid to cool.

Types of Reservoirs-1. Non-pressurized: The reservoir may be vented to atmosphere

using an air filter or a separating diaphragm.

The type most commonly used in industry, normally, has an air

breather filter, although in very dirty environments, diaphragms or

air bags are used.

i) Closed- No direct contact of fluid to atm air

ii) Open - Direct contact of fluid to atm air

2. Pressurized: Operates between 0.35 and 1.4 bar and has to be

provided with some method of pressure control; this may be a

small air compressor maintaining a set charge pressure.

Used in aircraft- because at high altitude Pressure is less

The advantages of a pressurized reservoir are that it provides

boost pressure to the main pump and prevents the ingress of

atmospheric dirt.

Filters /Strainer-

Foreign matter and tiny metal particles from normal wear of valves,

pumps, and other components are going to enter a system.

Strainers, filters, and magnetic plugs are used to remove foreign

particles from a hydraulic liquid and are effective as safeguards against

contamination.

Magnetic plugs, located in a reservoir, are used to remove the iron or

steel particles from a liquid.

Filters:

They are devices whose primary

function is the retention, by

some fine porous medium, of insoluble

contaminants from fluid.

Filters are used to pick up smaller contaminant particles because they

are able to accumulate them better than a strainer.

Types of Filters

1.According to the filtering methods:

a. Mechanical filters:

-Contains a metal or cloth screen or a series of metal disks separatedby thin spacers.

-are capable of removing only relatively coarse particles from the

fluid.

b. Absorption filters:

-Filters are porous and permeable materials such as paper, wood pulp,cellulose and asbestos.

-Paper filters are impregnated with a resin to provide added strength.

-The particles are actually absorbed as the fluid permeates the material.

Hence, these filters are used for extremely small particle filtration.

c. Adsorbent filters:

-Adsorption is a surface phenomenon and refers to the tendency ofparticles to cling to the surface of the filters. Thus, the capacity of such

a filter depends on the amount of surface area available.

-Adsorbent materials used include activated clay and chemically

treated paper.

2. According to the size of pores in the

material:

a. Surface filters:Simple screens used to clean oil passing

through their pores. The screen thickness

is very thin and dirty unwanted

particles are collected at the top

surface of the screen when the oil passes,

for example, strainer.

b. Depth filters:These contain a thick-walled filter

medium through which the oil is made to

flow and the undesirable foreign particles

are retained.

Much finer particles are arrested and

the capacity is much higher than

surface filters.

3. According to the location of filters:

a. Intake or inline filters (suction

strainers):

Provided first before the pump to protect the

pump against contaminations in the oil

Designed to give a low pressure drop, otherwise

the pump will not be able to draw the fluid from

the tank.

To achieve low pressure drop across the filters,

a coarse mesh is used.

These filters cannot filter out small particles.

b. Pressure line filters (high-pressure

filters):

These are placed immediately after the

pump to protect valves and actuators

and can be a finer and smaller mesh .

They should be able to withstand the full

system pressure.

c. Return line filters (low-pressure

filters):

These filters filter the oil returning from

the pressure-relief valve or from the

system, that is, the actuator to the tank .

They are generally placed just before

the tank.

They may have a relatively high

pressure drop and hence can be a fine

mesh.

Protect the tank and pump from

contamination.

Types Depending on the amount of oil filtered by a filter:

1.Full flow filters: Complete oil is filtered

Pressure drop increases as the filter

gets blocked by contamination.

2.Proportional filters

(bypass filters):

Only a portion of oil is passed

through the filter instead of entire

volume and the main flow is

directly passed without filtration

through a restricted passage

Hydraulic Strainers:

A strainer is a Coarse filter.

Fluid flows more or less

straight through it.

Constructed of a fine wire

mesh screen or of

screening consisting of a specially processed wire of

varying thickness wrapped around metal frames.

It does not provide as fine a screening action as filters

do, but offers less resistance to flow and is used in pump

suction lines where pressure drop must be kept to a

minimum.

Beta Ratio of Filters-

To calculate the filtration efficiency of a fluid filter

Beta ratio of 1= No particle above specified N are

trapped by the filter.

Beta ratio of 50 = 50 particles are trapped for every one

that gets through.

Most filters have a beta ratio greater than 75:

Ex-

β 10 = 10010 - particle size 10μm and larger

100- 100 times more particles upstream than

down stream which are greater than N

= Separation or Retention efficiency

Example:In a multi-pass filter test 40,000 particles of 10 micron and larger sizewere counted upstream and 8,000 particles in the same range werecounted downstream of the test filter. Find the beta ratio of the filter.

Intensifier used in hydraulic systems-For transforming hydraulic power at low pressure into a

reduced volume at higher pressure.

- Used to increase the intensity of pressure of any hydraulic

fluid or water, with the help of water or hydraulic fluid at

low pressure

Single acting Intensifier

Double acting Intensifier

Application of Intensifier

1. Pressure switchesA pressure switch is a form of switch that closes an

electrical contact when a certain set pressure has been

reached on its input.

The switch may be designed to make contact either on

pressure rise or on pressure fall

2. Temperature switchesTemp switches acts as protective devices

Is an instrument that automatically senses a change

in temperature and opens or closes an electrical

switching element when a predetermined

temperature level is reached.

1. The Thermostat

2. Resistive temp Detectors

3. The Thermocouple

1.The Thermostat-

Bulb thermometers are good for measuring temperature accurately, but they

are harder to use when the goal is to control the temperature.

The bimetallic strip thermometer, because it is made of metal, is good at

controlling things.

The principle behind a bimetallic strip thermometer relies on the fact

that different metals expand at different rates as they warm up.

By bonding two different metals together, you can make a simple electric

controller that can withstand fairly high temperatures. This sort of controller is

often found in ovens.

2.Resistive temp Detectors

3.The Thermocouple

3. Level sensors or indicators used in hydraulic

systems

Level Pro sensors are solid-state, electro-optic liquiddetectors having no moving parts. Utilizing the simple

physical principle of light refraction through a prism,

these sensors reliably and predictably signal the

presence of liquid with minimal hysteresis and without

the needs for periodic “zeroing” or calibration.