Download Experiment 2-3

Experiment 2-3
Fluid Friction
Fluid Friction
Objectives:
1. Demonstrate that all hydraulic components have an internal
frictional resistance to fluid flow which causes a pressure drop as
fluid flows through the component.
2. Explain how flow rate and orifice size affect the
fluid pressure drop across a hydraulic component.
3.Determine the pressure at any point in a fluid power
system based on the pressure distribution.
4. Determine the pressure drop between several points
in a fluid power system given the pressure gauge
readings at specific points.
5. Apply Pascal’s Law to a fluid power system in order
to determine the pressure at a given point.
Source of Fluid Friction
Frictional resistance to fluid flow comes from many sources.
First, the fluid itself will resist movement. Next, coming into
contact with any surface, be it in pipe, tubing, or hose, will
cause resistance. Resistance is unavoidable but can be
reduced to an acceptable degree with good design practices.
Reducing bends and turns in fluid conductors will have a
positive effect on friction as well as considering the internal
surfaces of components. A very important fact to remember
that resistance will cause energy loss in a fluid power system.
It is therefore necessary to discuss frictional loses in order to
appreciate system designs that minimize pressure drop. A
pressure drop is simply the difference in pressure between
any two points in a fluid power system.
Factors Affecting Pressure Drop
As stated in the lab manual, all components through which
fluid flows will cause a pressure drop to some degree. The
degree of pressure drop or “delta P” will be determined by
pressure, flow, and the inside characteristics of the component
to include its size, roughness, and shape. Fluid moves more
smoothly over smooth, contoured surfaces rather tan blunt,
rough surfaces. Sharp bends and turns should always be
avoided. The last statement may be hard to understand as
you notice certain equipment in which it may appear that
somebody disregarded this rule.
Hydraulic System Pressure Distribution
Operating pressure refers to the pressure at the outlet of the pump during
machine operation. Pressure at other points may vary greatly depending on the
movement of the fluid. While fluid is in movement, the pressure at any point will
be equal to the sum of all pressure drops from that point back to the pump outlet.
This rule follows the rule of series circuits in electricity.
Pascal’s Law Applied to Pressure Distribution
Pascal’s Law states the property of a fluid to transmit pressure equally and at right
angles to all containing surfaces. With this in mind, consider what happens when a
cylinder extends as far as it can go and then stalls out. While fluid is moving,
meaning that the cylinder is moving, pressure will vary from point to point.
However, after the cylinder stalls by reaching the end of its stroke, or encountering
a load it cannot move, the pressure at all points between the pump outlet and the
cylinder will be equal to the relief valve setting.
Charts and Graphs
A graph such as the one illustrated above, is used to show an existing relationship
between two variables where a predictable outcome may be established. In the
chart above, a valve is slowly closed resulting in an increase of pressure as
resistance to flow is increased.
Review
1. Explain the meaning of “delta-P.”
2. Give the delta P of A and C.
3. Name the characteristics of a component that affect the pressure
drop of fluid flowing through the component.
4. What is the pressure of the fluid at any point in a fluid power
system determined by? Assume the fluid is in movement.
Review
5. In the circuit below, what is the value of gauge S, or in other
words, what is the relief valve set at?
Review
6. In the circuit of question 5, what is the pressure at the cap end of
the cylinder? Assume the fluid is moving.
7. In the circuit for question 5, what is the pressure at the cap end of
the cylinder if the cylinder stalls out?
8. Why do you suppose that one component might have cause more
delta P than another?
9. In the illustration above, what happens to delta P if the pressure is
increased but all else remains the same?
10. In the illustration for number 9, what happens to delta P if the
needle valve is opened and pressure from the left remains the same?