MEC-E5003 - Fluid Power Basics, 07.01.2019-14.03.2019
This course space end date is set to 14.03.2019 Search Courses: MEC-E5003
Group Work Assignments
Group Work Assignments
Solutions (1-3) available
Group Work 3
The Powerpoint file was updated on 20.3.2019.
Reason: Parameter R (laminar valve resistance) is common for both systems (proportional and DDH).
Exercise 9
IF flow is turbulent , remember to use relative pipe roughness parameter, since the friction equation needs an input (roughness) parameter without unit [-].
Exercise 10
Proportional valve controlled system
This system's pump flows might be slightly easier to solve than the DDH system's. You can start with this and learn some new skills by solving this at first.
Attention!
Parameter R (laminar valve resistance) is common for both systems (proportional and DDH).
DDH system
In real world applications we must have a pump/motor which has the case drain leakage line to prevent high pressures acting against shaft seal and/or high pressures in the pump/motor case. The case drain must be connected to a low pressure source/sink -> tank or pressure accumulator.
Because of pump/motor leakage we lose fluid from the cylinder circuit and we must give it back and in this system we use check valves for that purpose. If the cylinder chamber pressure is small enough compared to accumulator pressure (0.5 bar gauge, remember also check valves cracking pressure!), check valve opens and fluid will flow to the cylinder chamber. Without the check valve we would face cavitation in the cylinder chambers (or there would be flow to the cylinder through the pump case drain line, which would not be desirable).
In this system architecture one of the cylinder chamber pressures are probably quite low, because the accumulator pressure is (must be) so low.
For system calculations
- Make hydraulic force balance equation for the cylinder.
- Use continuity equations for A and B sides/chambers, make equations for A and B flow rates
- Some of the flow rates may be be common for A and B sides -> needed to be able to solve the cylinder pressures and also the system flows
- Think, which check valves might by open and which probably not
- Think, in which laminar leakage orifices you have flow and into which direction
Group work 2
Usage of absolute pressures
Assignment 2 updated (12.2.2019)
Hints!
1.
Get new inspiration from "calculation exercises".
2.
You
can notice "essential similarities" between this assignment (#2) and the
Simulink - Simscape assignment. The cases (the systems) are NOT totally the same ("quite or very close") and
you can think/ponder what are the differences. However, you can get some
inspiration and confidence by examining the Simscape solution if you
compare the operation and results.
You
can add a gravitational load by using for example "Ideal Force Source",
"Simulink-PS Converter", and "Constant" blocks to your Simscape
application of you want.
It
must should be emphasized that the Assignment solution should be
carried out by using analytical means, by deriving equations and solving
them numerically! Simscape can be used to strengthen your faith and to
learn more.
Far below an example of the way the gravitational load (mg) can be added to the cylinder system.
You can find the blocks by a) double clicking Simulink canvas and b) writing part of the block name.
Blocks:
- Ideal Force Source (Foundation Library /Mechanical / Mechanical Sources -library)
- Simulink-PS Converter (Copy - Paste, old design in the figure), and
- Constant (from Sources library)
3.
Remember that
- cylinder is also a pressure transformer (force balance equation)
- cylinder is also a flow rate transformer (continuity equation binding A and B flow rates and piston velocity together)
Create
- Force equation (cylinder)
- Continuity equations (cylinder chamber flows and piston velocity)
Use
- Turbulent throttle equation(s)
- Boundary conditions are pump and tank pressure:
- describe throttle flows as functions of pressures (pressure differences) AND piston velocity
- you can have equation for force balance as a function of
- piston velocity,
- command signal (U), and
- pump pressure.
- Solve for the piston velocity.
- After that you can solve all the other variables.
Other solution methods also possible!
Page 6 (used to be page 5), InputsValve capacity
"valve capacity, full opening (40 l/min @ 35 bar)"
This means that
- with full opening (+/-10 V )
- with 35 bar pressure difference
- 40 l/min flow rate through (one throttle)
-> if you know
- valve command signal
- pressure difference
- you can calculate the flow rate
OR
-> if you know
- valve command signal
- the flow rate
- you can calculate the pressure difference