Rigid yet Flexible Coupling

Oct. 6, 2005
Road induced vibrations cause NVH problems in components especially those suspended off the vehicle frame. Harmonic vibrations can be destructive unless the components are rigidly coupled to the vehicle. Rigid connections eliminate or reduce random harmonics and match the vehicle NVH properties so that, they are not felt by the operator.

Steering Column Shaft Bearing Mount

William Rodgers
Mechanical Engineer, New Product Development
Pennington, NJ

However rigid coupling can also be a problem typically requiring heavier duty and more costly precision components & more difficult assembly. Loose fitting parts can result in noise or lead to premature failure.

The steering column is one such vehicle component that is relatively large, heavy and suspended off the vehicle frame. The column shaft is supported on bearings that allow smooth operation. Random vibrations from the tires on the road not matching the vehicle NVH properties are transmitted up the steering shaft to the steering wheel and felt directly by the driver. Making the column assembly rigid and strong enough to prevent random vibration typically adds to the unit weight and cost. If the coupling assembly is loosely damped the effects are typically worse with random vibrations and high amplitude causing premature bearing failure as well as movement and possibly noise in the overall steering system.

A major European steering column manufacturer was faced with these issues when designing a column for the VW Sharan 2003 (This platform also is used on the Ford Galaxy and Seat Alhambra with total European volume of approximately 100K annually) Design constraints were such that hand assembly of the shaft bearing was also required. They attempted to use rubber bushing to mount the precision ground bearing to the steering shaft and provide the necessary damping. Since rubber alone would not provide the support needed, a steel-reinforcing sleeve was added to the bushing and greatly increased the cost. The rubber steel bushing did allow hand assembly required in this application but did not give consistent performance, especially over the broad temperature range -40C to +85C°. Also the rubber deteriorated over time and failed.

Another solution was needed that would allow hand assembly, provide consistent performance over the life of the vehicle, improve or eliminate random harmonic vibration and yet remain cost effective. That solution was a novel use of a Tolerance Ring (manufactured by Rencol located in Bristol, UK an affiliate of USA Tolerance Rings in Pennington, NJ) designed to match the design constraints of the application.

In this application the design of the Tolerance Ring required a 1.05mm wave height to fit within the existing clearance between the bearing and the shaft. It was made of .2mm thick stainless steel strip and a 5.0mm wave pitch to handle the steering column system radial loads and support the steering shaft. Still further, it had to allow hand assembly and provide a flexible coupling between the bearing and the shaft. To accomplish this the Tolerance Ring curling process was modified to give a slight angle on the sides of the ring. That provided a significantly lower spring rate achieving the required hand assembly. Over the compression range of the wave the ring then demonstrated both a Rigid yet Flexible spring rate. Assembly was aided by the flat washer to back up the ring, holding it in position while the bearing was slid onto the ring. This made it possible to use of the existing shaft without any added cost for secondary machining. (See spring rate chart)

Subsequent testing of the steering column and mounting system shows that with the rubber steel reinforced bushing random harmonic vibrations occurred at around 30 Hz with a high amplitude (m/s2)/N. Repeating the test using the proposed BN30X10S1 Tolerance Ring revealed that the shaft and bearing were coupled rigidly such that the vibration frequency of bearing mount now matched the vehicle NVH properties in the 52 Hz range and the amplitude of the vibration was reduced by about two thirds. (See chart comparing vibration test with TR and with rubber bushing from the video)

Further development through the joint partnerships in design led to the use of a low cost un-ground bearing with bore tolerance of ±.1mm and shaft tolerance of ±.05mm since the Tolerance Ring allowed for the broad range of tolerance. Since the amplitude experienced with the Tolerance Ring was so reduced the steering column support frame no longer needed to be as strong and it too was redesigned to reduce weight. Overall the cost savings were significant & measurable; lower cost bearing, lower cost Tolerance Ring vs. the rubber-steel reinforced bushing; and lower cost, lighter weight vehicle support frame. Although the Tolerance ring allowed for the additional cost saving, the major advantage of the Tolerance Ring in this application was still the altering of the vibration frequency with consistent & improved performance over the life of the vehicle.

Tolerance Rings have been proven to be effective in various NVH applications ranging from simple fixing of a throttle body motor in an aluminum housing, to one like the above example where the frequency can be altered to improve component life and reduce noise. The Tolerance Ring is a precision-engineered device made form thin spring-steel strips of material into which waves, corrugations or bumps are formed. The strips are cut to length and curled into the ring shape. The waves are either facing inward or outward to accommodate different applications. The AN style, waves facing inward, is designed so that the ends of the split ring can be squeezed closed and fitted into a bore or tube and when released it is self-retaining. The BN style, waves facing outward, is designed so that when the ends of the split ring are opened it will slip over a shaft and again will be self-retaining when released.

The waveforms are designed to exert a holding force yet allow for ease of assembly between mating components. This closed end wave profile is typically very strong with the strongest part being the shoulders or closed ends of the wave. When the Tolerance Ring is assembled between mating components each wave is elastically deflected. Using the elasticity of the spring material then provides the holding force. The holding ability of the ring is the resultant radial force of all the waves and the coefficient of frication of the mating components.

Within the elastic limit of the wave configuration simple spring theory applies. The major factors influencing spring rate are strip thickness and wave pitch. By varying these, a wide range of spring stiffness can be designed into a given component envelope. Typically a range of greater than two order of magnitude is achievable within the same space envelope.

Spring Rate Chart

Fa (Retention Force) = CFr
     C = Coefficient of friction
     Fr= Spring Force Radially

Fr (Spring Force Radially)= KX
     K = Spring Rate
     X = Wave deflection

K (Spring Rate) = 4.8 x E x W x (T/P)3

     E= Elastic Modules for the ring material (KNmm2)
     W= wave width (inch or mm) - not to be confused with ring width
     T = strip thickness (inch or mm)
     P = pitch (inch or mm) 2

Typical Compression Ranges in Different Applications:

Bearing Mount - 0-9% could go higher to accommodate greater thermal affects.
Torque Transfer - 8-16%
Torque over load - 14% - 20%
Axial Slip - 16-35% with non STD wave profile

For more information on other applications of how a Tolerance Ring may benefit you in your application go to www.usatolerancerings.com or call Toll Free 877-865-7464.

USA Tolerance Rings
85 route 31 North
Pennington, NJ 08534
Toll Free 877-865-7464
Fax: 609-745-5012

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