More effective inspections and problem diagnoses can be made by"listening"' to these high-frequency sounds. By JOHN FREEMAN
The observation that truck components are growing in complexity would not surprise anyone closely associated with the industry. This growing sophistication is largely the result of the introduction of electronics, larger vehicles and government regulations. To keep pace with the equipment, a knowledge of what just a few years ago was considered to be space-age technology is becoming required in many fleet maintenance facilities.
Prior to 1964, when the nation's space industry was almost the sole user of ultrasonic detection equipment,no one in the trucking industry could have realized that these units were later to become "tools of the trade." Today the devices are being employed nationwide in maintenance shops to check and inspect such items as: internal combustion engine valving and piston blow-by; gaseous piping and ducting; air brake systems; bearings; cam and tappet assemblies; gear trains; seals in refrigerated van bodies, clean rooms and air ducts; various hydraulic components and many other units.
"Ultrasonics" is a word and an area of technology that some shop managers might still consider to be rather foreign. This remoteness, however, is mostly just imagined and certainly not something to be feared. Except for their frequencies, ultrasonic waves are exactly the same as their audible counterparts. In fact, many lower frequency, "sonic'' waves possess ultrasonic counterparts. The hissing of leaking pressurized air, the noise of jet planes and the jangling of keys are all examples of common sounds that are often heard but also contain unsuspected ultrasonic frequencies which cannot be heard. Many people use sounds to detect problems of one sort or another on a regular basis. The watch repairman listens to the tick of a watch. The plant engineer listens for the sound of escaping steam.
Because ultrasonic waves are not as penetrating as those of lower, audible frequencies, it's generally easier to pinpoint the source of the sound. Moreover, there's usually much less noise interference. In particular, conversation and most background noises are normally absent. Extremely small sounds can, therefore, be detected and analyzed.
This article will describe some of the tests which can be performed with an ultrasonic analyzer, but first a brief discussion of the instrumentation is in order. The detector is basically a microphone and associated electronics, which is sensitive only to ultrasonic energy in the frequency range of 35,000 to 45,000 Hz. These signals are amplified by the self-contained circuitry and converted either to sounds that can be heard by the mechanic or readings on a meter. The most useful type of instrumentation is designed specifically for the type of tests done on fleet vehicles. It includes a contact probe which can be used in place of the microphone, a separate ultrasonic generator that is used to check for faulty seals in cabs or trailers and some sort of device which can help localizethe source of air-borne sound.
When properly used, an ultrasonic detector can help pinpoint leaks quickly in both vacuum assisted and air brake systems. In many cases, leaks can be detected from a considerable distance. A systematic procedure usually works the best in this application. After building up full air pressure, shut off the engine to eliminate interfering exhaust and mechanical noise. A casual sweep of the vehicle from one end to the other using the detector with a hand held microphone can often locate the leak. When necessary, headphones can be used to reduce noise interference from within the shop. If the first sweep does not locate the teak, trace out the entire air system piping at close range (one to two feet) with the hand probe. Once the general area of the leak has been determined, the sound concentrator or localizer can be used to pinpoint the exact location of the leak. The sound concentrator is usually a small rubber funnel which reduces the sensitive area of the probe to a circle of about ¼ inch (6.4 mm) diameter. The device also sharply reduces the effect of ultrasonic noise originating away from the area of interest.
For effectiveness, be sure during the entire search that pressure is maintained in the brake system. If no leaks are discovered, check the shutter-stat, windshield wipers, air horn and other air-actuated equipment. Leaks in these accessories can produce abnormal gauge drop.
When it's necessary to perform tests with the engine running or in the presence of other sound sources which interfere with the use of the open hand probe, the sound concentrator in most cases locates the leak by shutting out most of the interference. It does, however, require much closer and more careful tracing of potential leak points.
Air escaping from pressurized systems generates ultrasonic frequencies which can be easily detected.
For heavy trucks, many mechanics check tires for adequate pressure by striking the tire with a hammer and noting the rebound and sound. This procedure doesn't find leaks. It merely finds soft tires. It's possible that a leak may have just occurred or the tire has just been inflated and hasn't had time to become soft.
A sweep with the hand probe may remedy the situation. The detector normally finds leaks of sufficient magnitude before they cause on-highway downtime later. The detection of even one leak may, very possibly, save the cost of original investment into the equipment. Facts which bolster this belief are savings realized by preventing the loss of a tire (on duals, a second tire), possible damage to driveline components and road service costs to replace the tire.
Leaking intake valves can be checked for defects by using the probe and the headphones with the engine running. All valves should emit a sound in a very close meter and sound pattern when the probe is placed on the intake manifold opposite the intake port. The valve or valves out of pattern can then be positively checked, when the engine isn't running, by bringing that particular cylinder to full compression and placing the probe on the valve stem. The "hiss" of escaping gas across the valve seat will be distinctly audible on the leaking valve. Exhaust valves can be checked by using the same procedure. However, signal intensity will be much greater in them.
Efforts to determine the presence of air in the cooling systems of fleet vehicles is receiving more and more attention, particularly in diesel engines. Air can cause hot spots resulting in burned liners and heads in a very short time. Since all cooling systems are under pressure, coolant must go out in order for air to get in.
Aeration detection equipment gives the signal that air is in the system but not the location of the leak. Using an ultrasonic probe in suspect areas such as the head gasket, around the water pump and injectors will usually detect the exact source of the leak. The "bubbling" of air in a cooling system is very noisy and usually enables the exact determination of the problem.
Being able to single out a certain unsuspected trouble point can save a lot of unnecessary labor. All such tests should be conducted while the engine is idling. Internal leaks may also be pinpointed with the probe immediately after engine shutoff when internal Pressure is at its highestpoint. Headphones should be used for this.
With the engine idling, pulsation in the exhaust system can be detected with the hand probe. Any leak points normally give off a much louder and sharper sound. If required, the sound concentrator can be used to locate the exact leak point. Again, headphones are essential for this procedure.
A ball or roller bearing that's in good condition and is adequately lubricated normally produces a soft "whirring" sound via earphones. Lack of lubrication introduces scraping sounds. Flat spots or nicks on the bearings or race cause a grating, grinding or clicking noise, depending on the speed of rotation and the degree of damage. For best results, it's recommended that the contact probe should touch the spindle. Spin the wheel with a spinner or by hand. If questionable sounds are heard, it's often possible to determine whether it originates from the inner or outer bearing. Simply contact the spindle on both sides of the wheel. Again headphones might be useful in a noisy maintenance facility.
Ultrasonic inspection has a definite advantage over audible inspection because ultrasonic waves don't propagate through equipment and compound themselves. Thus, the source of ultrasonic sound always gives off the strongest signal and can be accurately pinpointed. In many cases, a stethoscope tends to lead away from the sound source.
Using the contact probe with earphones, it's possible to determine which gear or bearing in a drive is faulty. This should be done by allowing the drive train to run with the wheels raised off the ground. The contact probe and headphones are the search combination to discover the point of greatest noise generation.
Many sounds that interfere when attempting to use a conventional stethoscope don't exist at ultrasonic frequencies. This also contributes to the overall effectiveness of the technique by reducing confusing background noises.
Partially blocked or bypassing valves in high pressure hydraulic systems can be extremely difficult and costly to locate. However, maintenance managers may find that an ultrasonic detection device is extremely helpful in spotting them by touching the contact probe to suspected fittings, valves and power cylinders. Since hydraulic systems employ very high pressure, the intensity of the sound given off by an internal leak is extremely high.
For most effective operation, amplification on the detector shouldn't be adjusted any higher than is necessary for a clear and distinct ultrasonic signal. Greatest sensitivity is obtained when the amplified sound can be barely heard.
When sweeping with the hand probe, don't move too rapidly. A signal source could easily be overrun. Also keep in mind that when using the contact probe, put enough pressure on the probe for a good contact. Be sure to place the probe in the same spot when making repeated tests on the same components.
