1. Thought for the day “Engineering is the art of modelling
materials we do not wholly understand
into shapes we cannot precisely
analyse to resist loads we cannot
properly assess in such a way that the
community has no reason to suspect
the extent of our ignorance”
President, Scottish Branch - Institute of
Engineering UK, 1946

2. Hydraulic System Overview - Norman Thornton, Engineering Consultant
A brief description of the basic system elements will be followed by the effect of performance when specifying amplitude, frequency combined with equipment.

3. Servo Hydraulics in the field of mechanical testing for performance
A fundamental need in all testing is repeatability which is enhanced by equipment operating in a stable and reliable condition. Changes should be to the specimen not the equipment doing the test.
Do we have enough power? Why do we need this power? What can we do with the power we have. How can we reduce power? What is limiting my predicted performance. What do we provide to improve the predictive modelling in Design.
Do we have a complete understanding of the test specimen in terms of material, manufacture and residual stress state.

4. BASIC HYDRAULIC CIRCUIT
lcu
pump
Controller
pressure
return
drain
Distribution manifold

5. Road Load Data (Force & Deflection)

6. Pump Flow Capacity Check

7. Test Rig Hydraulic Flow

8. Accumulator Flow

9. Actuator Flow Calcs (Sine Wave)
Pv (Peak Velocity for a Sine Wave)
Pv = 2 π x frequency x ½ amplitude
Pf (Peak Flow for a Sine Wave)
Pf = 2 π x frequency x ½ x area

10. Compressibility of Oil
As a general rule oil compresses at 0.5% per 1000psi (70 Bar)
Example:- This represents an equal area actuator operating at 210 bar
At null position, the pressure P on both sides of the piston is equal (105 bar)
The pressure drop across the servo-valve is 70 bar (∆p), therefore the maximum dynamic pressure available is 210 – 70 = 140 bar
At this pressure the compression volume = 140 / 70 = 2 and as a % = 2 x 0.5 = 1 %
And the dynamic compression flow Pcf = 2 π x frequency x ½ ampl. x (1% of stroke) P P

11. Actuator flow calculation

12. Actuator Performance Calculator

13. Considering The Actual Test
A number of factors can affect the reliability and repeatability of the test:-
INERTIA
REACTION OF FIXTURES (loads & deflections)
VELOCITY
BANDWIDTH
ATTEMPTS TO ACCELERATE A TEST

14. Considerations for Accelerating a Test
The definition of a “load cycle” and the decision whether it can be omitted or not is only possible if all simultaneous acting forces and the local stresses are taken into account.
Every change (shortening) of one individual channel requires an appropriate change of all other channels, due to existing correlations (phase Relations)
The shortened load-time histories should have qualities that allow us to exploit the capacities of the test rig as far as possible during the whole test without exceeding the given limits.

15. Vauxhall Vectra Suspension Components

16. Summary of parameters for Simultaneous Procedures
Load cycles of the resulting local stresses in a component being below a certain limit can be omitted e.g 15% of max stress.
The significant turning points of the local curve must be maintained.
The maximum deviations and frequencies of the load-time histories shortened should not exceed the given limits.
RESTRICTIONS
Inertia Forces – Application of Simultaneous Procedures is restricted to tests with test rigs and components that can be considered as STIFF.
Damping Elements – Forces depend on speed and frequency which are changed by Simultaneous Procedures.
Rotating Components – Not currently applicable.

17. Actual Component Effects on Test Performance
Material
Metallurgy
Residual Stress State
Design
Stress Concentration
Process
Machining
Welding
Heat Treatment

18. Process Control - Example
Three components:
•Steel from same supplier and factory
•One prototype assembly
•One assembly from production at Factory 1 (–50% of reference life)
•One assembly from production in Factory 2 (+50% of reference life)
Rear suspension arm

19. Process Control – Residual Stress State
Compression
Prototype
Tension
Factory 2 (+50%) life)
Factory 1 (–50% life)

21. SUMMARY
ANY COMPONENT TEST WILL BE IRELLAVANT IF YOU DON’T KNOW THE STARTING POINT:-
The specimen:-
Material, manufacture, stress state
The test specification:-
Quality and interpretation of the data
The test system:-
Fixtures, reaction points
Limitations of the servo hydraulic system
Repeatability of the test