Experiment 2-3 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?