Design by Objective: Vibration & noise

June 1, 2004
Excessive vibration and noise are not viewed as marks of a high-quality product. Motion System Design asked industry experts how to deal with the shakes and rattles

Define vibration.

WAJDA: Vibration is the oscillation of a body around a reference point. Vibration exists when a system responds to some internal or external excitation. Vibration is a dynamic response to a dynamic force, and an increase in force decreases bearing life. Doubling the load while maintaining a constant speed decreases a ball bearing’s L10 life by a factor of 8. Concentric adapter-mount bearings minimize vibration compared with setscrew and other non-concentric products.

O’CONNELL: Vibration causes noise, and the two are definitely related in ball and leadscrew assemblies. Although vibration specs are fairly straightforward, noise is subjective in that there is no industry standard for the acceptable noise level of a screw assembly.

LAY: Sound level is measured in decibels, but many engineers rely on perception rather than measured results. In gearing applications, for example, especially plastic gearing, noise level often means what an engineer actually hears. Here, what’s considered low noise can vary from application to application and from engineer to engineer.

What are limiting factors when trying to reduce vibration?

O’CONNELL: The biggest factors leading to excessive vibration are misalignments, as well as exceeding the screw shaft’s critical speed or the ball nut’s rotational speed. Drilling out or using a hollow screw shaft can reduce vibrations caused by running near the shaft’s critical speed, and compounds can be placed in the hollow shaft for additional vibration damping.

WAJDA: Limiting factors are clearance, looseness, non-concentric shaft attachments, and assembly constraints. The adapter-mount ball bearing compensates for tolerance deviations between the shaft and bearing bore, removing clearance and looseness to create a tight fit between mating surfaces. The adapter mount also helps center the mass of the rotating element at its center of gravity.

LAY: With respect to plastic gearing, most noise issues are related to improper gear design or too much backlash between the mating gears. Sometimes, greases with “adherence modifiers” or “tackifiers” can reduce the noise level associated with backlash. Other times, applying an oil or very soft grease to the bushings or shafts on which the gears move will reduce noise and vibration.

What should engineers know about component interactions and how they affect vibration?

LAY: Material selection can be key to reducing noise. Running two mating gears of differing materials (acetal on nylon) can have lower noise properties than two similar gear materials running together (acetal on acetal). However, to cut costs, some applications rely on like materials, and by using an external lubricant (grease or oil), noise levels can be reduced.

WAJDA: Typically a force unbalance will be in phase and steady. The amplitude due to unbalance will increase by the square of speed (3x speed increase = 9x higher vibration). A 1x rpm spike is always present and normally dominates the spectrum within an unbalanced condition.

O’CONNELL: It can be difficult to isolate the exact component causing the vibration and noise. Many times noise and vibration can be translated though other rigid members of the drivetrain. Noise generated from the ball nut assembly can be minimized with an alternating ball size, including plastic balls and spacers, or maintaining a vertical orientation of the return tubes on ball nut assemblies that use external return mechanisms.

What can engineers do to reduce vibration in their design?

WAJDA: Remember that vibration is detrimental to bearing life and machine performance. A system that incorporates concentric adaptation of components decreases system vibration and increases machine reliability. Looseness inherently creates problems with mating components and should be avoided when possible.

LAY: Many noise and vibration problems are exacerbated by increased wear. Select a quality lubricant early in the design stage to reduce these problems and shorten design time.

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