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Testing Rebuilt Power Steering Components: The "Zero Defect" System
By Pablo Aguilera
Defective returns. These two words have many undesirable connotations to the power steering rebuilder. Starting with a replacement part request, other claims could be forthcoming. These range from labor for removal and reinstallation, which can sometimes exceed three hours, to and including alignment, possible towing charges, rental car and even product liability expenses.
These claims only address the initial economic impact of this defective part scenario because you also have to consider the customer dissatisfaction that repetitive defects generate. Customer dissatisfaction always impacts future sales.
It is therefore imperative for the rebuilder to implement a quality assurance program that will minimize defective returns and their negative impact on the company’s bottom line, growth potential and reputation. A quality assurance program is not a singular but a multifaceted approach used to accomplish this goal. It is comprised of many interrelated elements that need to be implemented in sequential fashion to achieve the desired but elusive "zero defect" condition.
Key internal elements in a quality assurance program include: proper disassembly, cleaning, inspection and machining procedures; meticulous assembly using high quality replacement parts; and a thorough and complete testing program for defect detection/correction.
Externally, there are other elements to reduce and prevent defective parts claims that should not be overlooked. These can include providing installation instructions including system-flushing methodology, flyers and even clinics to educate your customers and prevent installation related errors.
Of the various elements of a quality assurance program, we are now going to direct our attention to the critical testing phase for the rebuilt power steering product. My company, P.S. Systems, Inc. has been rebuilding all types of power steering components for more than 15 years in the South Florida area. We also design and manufacture test equipment for the rebuilding industry and offer start-up training programs for those companies wishing to initiate or diversify their product lines. We are also an authorized rebuilder of CV axle products.
The primary goal of testing is to verify that the original parts, new parts being used and labor expended by the rebuilder, result in the satisfactory performance of the rebuilt component. When dealing with hydraulics, minor imperfections in the parts or lapses by labor can render a unit unusable or unsatisfactory in performance immediately or prematurely during the warranty coverage period. It is therefore imperative that every unit rebuilt is tested to ensure that the proper operating parameters are being met and satisfied.
How is this best accomplished? Through operationally simulated testing which attempts to duplicate the operating conditions found in the vehicle. How operationally simulated testing is conducted by our company using the test equipment that we have developed for this purpose for power steering pumps, rack-and-pinion and gearboxes is described in the following paragraphs.
In testing, a systematic approach will improve the chances of finding problems, decreases the time to correct them and ultimately improves customer satisfaction. Remember, when trouble shooting a system, complete diagnostic inspection must be emphasized to avoid overlooking problems that may not be obvious or difficult to detect.
Pumps: the energy source
The high performance hydraulic pump is the power source of the power steering system. Its performance, or lack thereof, at various rpm levels will determine how the system performs. It is therefore imperative that the test sequence be conducted at various rpm levels to guarantee faultless system performance.
Our pump test unit is designed to satisfy this requirement by using a programmable speed control that is mated to a rotation direction switch (clockwise/ counter clockwise) and a four-position switch for fast and repeatable rpm changes during the test sequence.
Since the maximum steering effort occurs during slow speed maneuvering, as in a parking situation, the idle pressure (psi) and flow (gpm) test becomes critical for proper steering response. These slow speed tests should be conducted at a speed ranging from 500 to 750 rpm.
Verification must be made that the minimum acceptable flow rate is present at these speeds for a given pressure level. For heavy-duty pumps, particularly of the diesel-driven, low rpm variety, a high output pump test model has been developed to accommodate their high power requirements.
Another testing parameter that is particularly important in automotive applications is noise. The operational test for noise should be conducted at higher rpm since this condition is normally amplified as speed increases. This necessitates an enclosed cabinet for drive motor noise suppression. As the speed is increased, additional hydraulic parameters are checked to determine pressure relief values, pressure regulator cycling response and flow rates. These tests are typically conducted at 1,500 to 2,500 rpm.
Of particular concern during the initial pump test start-up sequence is test fluid aeration. This can slow the test, result in erroneous hydraulic readings and increases pump cavitation/noise.
Our tester uses a pneumatically activated diaphragm pump to pre-lubricate and prime the system. For those cases where cavitation occurs, an air bleed-off system is provided that allows trapped air to escape.
During the test and at its completion, the pump should always be tested for external leakage prior to removal from the stand. One of the factors that can affect this leakage potential, as well as noise is the side load being generated on the pump shaft by a tensioned belt driven pulley. Since this is the typical configuration found on most vehicles, we designed our tester to duplicate the correct belt tension using a novel adjustable pneumatic loading system.
This system allows quick connection and disconnection of the belt from the test pump and variable belt tensioning adjustment through the use of a conveniently mounted pressure regulator. With our quick installation pulleys (no pressing on/off), the belt installation procedure is very fast. The photo on page 39 shows our high output pump tester with the expandable hydraulic option ready to initiate a test sequence for a heavy-duty pump and gearbox.
Rack-and-pinion and gearboxes
Whereas pressure and flow rate are of primary importance in pump testing, other variables become salient during the testing sequence for racks and gearboxes. Since we are dealing with the transfer of power at various resistance levels and speeds, mechanical input and output forces must now be addressed.
These forces are both rotational and linear in nature. For the input side, we now have to consider the input force (torque) that is applied to induce a steering response. On the output side, for racks the force is linear and for gearboxes the resultant force is rotational in nature.
It is imperative that the test machine being used is capable of handling these diverse power transfer requirements and diverse equipment mounting configurations of these components. The photo on page 38 shows how a gearbox is mounted for simulation testing in our rack-and-pinion and gearbox test unit
There are a multitude of tests that can be performed to verify the proper operation of these steering components. They range from very simple hydraulic tests with no simulation resistance - such as internal and external leakage, control valve balance, returnability, etc.
With simulation resistance more advanced tests can be conducted including piston bypass, input shaft and torque analysis (drive effort tests). More sophisticated tests such as wet and dry mesh tests (pinion torque), wet and dry return, valve effort and balance, and internal leakage tests at specified pressure levels are available. However, these require automated features as described in the next section.
For both of these components, test fluid recuperation following the test is an important cost saving feature that we accomplish using an air flush/test system.
Automation: the OEM approach
While automation might not be a cost effective test method for most rebuilders, it is of paramount importance from the OEM perspective. The purpose of automation is not to simply speed up the test cycle.
Benjamin Lanning, Delphi Saginaw OEM aftermarket engineer states, "Delphi Saginaw Steering Systems demands a high level of quality of our products. The integration of automation into almost every aspect of our manufacturing process has helped us achieve this more efficiently. The most important stage of our manufacturing processes is product testing. Every power steering product that we produce and remanufacture is tested for proper operation against our strict quality product specifications."
So how does automation increase the efficiency and quality of the test procedure? There are many ways and some of the most important ones are indicated below:
The test cycle speed is optimized and since the machine performs the test, the operator can prepare the next part for test to further reduce test cycle time. The machine also determines pass/fail using color-coded screens (green-pass, red-fail) so operator error potential is eliminated, improving test quality. Of course, excellent test reproducibility and repeatability is also achieved.
Furthermore, rebuilders have the ability to retest any of the specific test parameters. Because of its computerized nature, Windows software makes operation straightforward, as well as provides network and diagnostic capability. Another benefit is the ability to provide data storage and generate unique serial numbers for "passed" parts.
These are some of the advantages of automated testing. The disadvantage is cost, which can exceed by three to four times the cost of a manual unit. To date, we have designed several automated power steering testers and are presently in the pre-construction phase of a pump tester. These units will be computer controlled and are custom built to the customer’s exacting requirements.
Variable assist and other controls
Variable assist is a method of reducing the available steering assist as the vehicle speed increases. This provides the driver with a firmer and more controllable steering feel at higher vehicle speeds. The first units to incorporate this feature were hydraulically controlled by a mechanically driven speed-sensing device.
As the technology developed, electronic controls were developed for the power steering pump and rack-and-pinion units to reduce assist. These control mechanisms are typically found in the higher priced models and some also contain feedback sensors for fault detection. They are DC powered and current controlled (milliamp range). In a future article we will discuss in detail electronic control units (from simple E.V.O. to Magnasteer) and the testing methods and procedures for correct performance verification.