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Vehicle Speed Related Vibrations
Engine Speed Related Vibrations
Automobile and truck vibration diagnosis has been a trial and error process for many years. Service technicians in the field often fought over who had to work on the vehicle with a vibration rather than who got to work on it. With a little bit of training and understanding, what used to be guesswork and frustration can be made into an exact science and a money maker.
To begin with, there are a few concepts to understand, and a few terms to familiarize yourself with.
What is a Vibration? - A vibration is a repetitive motion back and forth relative to a fixed central position (Oscillatory Motion). This resulting motion can be felt, or heard, or both depending upon the speed of repetitive motion and the severity of the force which causes the repetitive motion. There are two categories of automotive vibrations:
Forced Vibrations - A forced vibration is typically caused by a rotating object. The object is being forced to rotate by a power source such as the vehicle's engine, an electric motor, or the momentum of a moving vehicle. The vibration will remain as long as there is a source of power causing the object to rotate. The motion can be in any direction, but is typically at a right angle to center shaft of a rotating object. For example: A tire which is out-of-round or out-of-balance will typically have a force which causes an up-and-down motion relative to the axle rather than a side-to-side or diagonal motion. The amount of motion is relative to the rotational speed and the amount of imbalance or out-of-roundness of the rotating object. This is the most common type of automotive vibration.
Free Vibration - An free vibration is typically caused by a moving vehicle coming in contact with a bump, crack, or disturbance in the road surface. This bump, crack, or disturbance in the road can cause a momentary vibration in components like the suspension system, exhaust system, body sheet metal, sun visor, steering wheel, etc. This type of vibration typically dies out quickly and can no longer be felt or heard. This is the least common type of automotive vibration.
All automotive vibrations have three basic components:
Source - The source of the vibration. Most of the time the source of the vibration is a rotating component that is out-of-round or out-of-balance. In these cases, the source of the vibration can be repaired. Engines however, have vibrations which are a natural part of the engine running. If the engine is the source of the vibration, it cannot be repaired because it is not broken. Instead, the transfer path for the vibration must be repaired.
Transfer Path - The transfer path can be anything that is connected to, or touches both the vibration source and the responding component that the driver notices or feels. It can be the suspension system, a motor mount, transmission mount, body mount, exhaust system, propshaft, running boards, etc. Whenever normal engine vibrations are felt in the passenger compartment, the transfer path should be repaired rather than the source.
Responding Component - The responding component is the part that the driver notices or feels is vibrating. It can be the floor, the seat, the steering wheel, the sun visor, the dash or instrument panel, etc.
The following terms are required learning if you wish to understand automotive vibration diagnosis, or if you wish to speak intelligently to another person about automobile vibrations. These are simplified definitions intended to apply to automotive vibration diagnosis.
Cycle - One complete disturbance or shake.
Frequency - The number of cycles that are heard or felt in a certain amount of time. The unit of measurement for vibration frequency is typically Hertz (Hz), indicating cycles per second; however, RPM, indicating cycles per minute, can be used also. The advantage of using RPM rather than Hz is the ease of matching the RPM of the vibration to the RPM of a rotating component. For example: If the vibration frequency measured is 2000 RPM and the engine RPM is also 2000, it is easy to see that they match. However, if Hz is used, a vibration frequency of 33.33 Hz is not an obvious match for the 2000 engine RPM. Some vibrations can be felt, but not heard (Typically frequencies lower than 20Hz which correspond to the speed of most tires), and that some vibrations can be heard, or felt and heard (Typically frequencies in the 20Hz – 20,000Hz range).
Amplitude - The measurement of how hard or harsh a vibration is. Low amplitude vibrations usually can not be felt or heard. High amplitude vibrations can be heard or felt or both depending on the frequency of the vibration. Decreasing the amplitude level of a vibration concern is the primary objective in the repair procedure. On the EVA's live data screen, a the lower the G's number, the milder the vibration. Typically there is not a specification for how low of amplitude level is acceptable; you must take before and after amplitude readings to verify that your repairs were worthwhile.
Natural Frequency - The frequency that an object vibrates the easiest at. An example is a piano tuning fork or an open ended hand wrench, if you tap the open end on a hard surface, it will emit a tone. A microwave oven vibrates water molecules at their natural frequency, 2.4GHz, to get them to vibrate and heat up. The natural frequency of automotive suspension systems has typically been between 10 and 15 Hz; however, later suspension designs have raised the frequency to 20 - 25 Hz. Some luxury sports cars have a natural frequency of as high as 49.5 Hz.
Point of Resonance - The vehicle speed or engine RPM where the amplitude levels from two or more frequencies (one will be a natural frequency) will add together creating a much harsher vibration at that point than at any other point. If the vehicle speeds up or slows down, the vibration will change or even go away. For example:
Older design automotive body and suspension systems have a natural frequency in the 10 - 15 Hz range. This lower natural frequency allows the point of resonance from tire speed related vibrations and suspension natural vibrations to appear at highway driving speeds (55-65 MPH). This means a tire which is slightly out-of-balance will be noticed quite easily at the point of resonance.
Many of today’s automotive body and suspension systems have been designed to have natural frequencies in the 20 - 25Hz range. This higher natural frequency raises the point of resonance from tires speed related vibrations and suspension natural vibrations to well above the legal speed limit. This allows for a tire which is slightly out-of-balance to not be noticed as easily.
Order - The number of disturbances or shakes in one revolution of a rotating component. Certain orders of the vibrations can only be caused by certain failures, for example: Anything that is out of balance will only cause a first order vibration. In other words, balancing a tire with a second order vibration is a waste of time. Understanding this concept can be very helpful when diagnosing the source of the vibration.
One shake per revolution is referred to as a first order vibration, two shakes per revolution is referred to as a second order vibration, three shakes per revolution is referred to as a third order vibration, four shakes per revolution is...
There is no limit to the numerical value of order; however, the higher the order, the less likely you will ever see it.
Order values can also be real numbers having decimal values like 1.5 order, 0.5 order, 1.23 order, etc. They do not have to be Integers like 1, 2, 3, etc.
Anything that is out of balance will only cause a first order vibration. Any higher order vibrations are not caused by an out-of-balance component.
Damping - The ability of a damper to change the frequency of another object either by adding additional mass or weight to the object, or by providing a cancellation of the objects vibrations.
Mass Dampers (Big Heavy Weight) add mass or weight to an object to slow down or decrease the vibration frequency.
Dynamic Dampers (Rubber Mounted or Viscous Fluid Mounted Big Heavy Weight) are fine tuned to cancel the vibrations of a specific object. This cancellation usually only applies to a certain arrow range of frequencies.
Total Indicated
Harmonic – in a harmonic measurement, both the magnitude and the duration (length) of the measured segment are used.
Total indicated reading (TIR) - the maximum dial indicator movement (peak-to-peak) during one revolution or cycle.
There are two types of total indicated readings:
Loaded TIR of the tire is used since it is compressible and changes under force.
Unloaded TIR of the wheel is used since it is not compressible and changes little, if any, under force.
In the past, some vehicle manufacturers have published unloaded TIR specification for the tire/wheel assembly. Total indicated readings in the tire/wheel assembly alone do not account for the duration (area) of the occurrence (bump), and can not determine the magnitude of the force as displayed in a harmonic.
The illustration below shows the limitations of using TIR for vibration resolution in comparison to First Order Tire Speed related vibration harmonic (R1H) measurement. The TIR is .040” in both examples, but the durations are not the same and therefore the harmonics will have different effects and completely different values. The example below may have the same TIR but the T1 harmonic could be 80 pounds of R1H harmonic force on the left versus 5 pounds of R1H force on the right. The HUNTER GSP 9700 can calculate these forces.
The above Drawing and TIR information is courtesy of www.hunter.com
Now, return to the menu at the top of this page and select the next topic; Vibration diagnosis Tools. Continue reading through each topic until you are finished. If you have any questions, email me at jkelly@vibratesoftware.com, or call me at 1-801-791-5807.
John D. Kelly
Address: P.O. Box 3065, Ogden, UT 84409-1065
Phone 1-801-791-5807
E-mail: jkelly@vibratesoftware.com
1998 - Master of Education in Curriculum and Instruction - Weber State University
1993 - Bachelor of Science in Electronic Engineering Technology - Weber State University
1990 - Associate of Applied Science in Automotive Service - Weber State University. WSU has one of the finest Automotive Programs in the country.
Twenty-nine years in the automotive industry.
2006 - Today - Chairman of the Automotive Technology Department at Weber State University.
2004 - 2005 - Assistant Professor in the Automotive Technology Program at Weber State University.
1991 - 2004 - Thirteen years as an instructor at the General Motors Training Center at Weber State University.
1980 - 1991 - Twelve years as an automobile technician working for various independent repair shops and General Motors Dealerships in Utah and California.
GM World Class Master Technician Certification.
ASE Master Automobile Technician Certification.
ASE L1 Certification.
Toyota Certified Expert Level Technician in Engines, Chassis, Drivetrain, and Electrical Systems.
Chrysler Level 4 certifications in Automatic Transmissions, Brakes, and Climate Control Systems.
Honda Certified in General, Express Tech, Pre-Delivery Inspection, Electrical Fundamentals and Air Conditioning.
Member of Society of Automotive Engineers (SAE International)
Member of the International Association - General Motors Automotive Service Educational Program (IAGMASEP) curriculum committee.
1998 - 2008 - Designed, marketed, sold, and supported customized automobile and truck vibration diagnosis software. The latest version of the software is currently used worldwide by General Motors Corporation, Ford Motor Company, Freightliner Truck LLC., and Mazda USA in their training Centers for vibration diagnosis training. The use of the software is also called for in their service manuals and is used by their dealership technicians for vibration diagnosis. The software is sold worldwide by SPX corporation, Kent-Moore Tools of Warren Michigan, Rotunda Tools of Dearborn Michigan, and by my company's website www.vibratesoftware.com.
1997-2004 - Participated as a Subject Matter Expert in the curriculum development for the following General Motors training courses: Automatic Transmission Diagnosis, Base Brake Systems, and Vibration Correction.
2003 - Authored Book: Specialized Automotive Electronics Training - Weber State University Printing
2002 - Wrote, designed, and currently teach four online courses for Weber State University in Automotive Electronics.
Currently writing a book on automobile and truck vibration diagnosis.
This page was last modified Thursday, March 05, 2009 09:02:10 PM
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