Application Digest

It is said for every problem there is a solution. The design options available through plastic rolling element bearings clearly confirm this: Integration of gears, shafts, and housings turns a bearing into an assembly, saving parts and labor.

Expertise in developing this technology is based on the ability to analyze the application, rather than the moving element within it. Not only is it important to understand the speed at which the bearing assembly will run, for example, but also running time and dwell time.

Main bearing for police car rotating reflector incorporates worm gear and fixing clips.

Material selection, moreover, depends on performance requirements and operating environment. Steel bearings have problems running in liquids, chemicals, and even dry situations where contaminants such as paper dust are prevalent. These areas can be addressed when designing plastic bearings through specific ball types, raceway material, and the fact that plastic does not corrode. Other design considerations:

• Engineering polymers offer higher temperature limits than previously available.

• Dimensional stability. Most engineering polymers are significantly affected by thermal expansion and fluid absorption.

• Size constraints. Dimensional accuracy and adequate support for bearing loads typically limits bearing OD to 11 mm minimum, 150 mm maximum.

• Loads. While plastic bearings cannot withstand the same loads as steel bearings, there are ways to increase load capacity, including increased ball size and pitch circle diameter, designing a full complement bearing, or incorporating two ball rows instead of one.

To speak with a BNL applications engineer, call 617-575-0737.

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Designing components for water hydraulics

by Torben Pedersen, Design Engineer Danfoss A/S Nordborg, Denmark

Water offers a number of advantages over hydraulic oil. It is environmentally friendly, non-flammable, and clean. In addition, fluid costs--in terms of initial fill, replacement, storage, and disposal--are inexpensive.

Once the designer decides to use water as a hydraulic fluid, however, some important properties of water must be addressed. Like oil, water is a Newtonian fluid, and the same physical laws apply to both media. However, water differs from mineral oil in three important ways: It has higher density; lower viscosity, and poorer compressibility.

Consequently, it is important for designers to be aware of the following issues:

• Water's density and compressibility causes an increased tendency of water hammering in quick closing valves and long pipes. In long runs of rigid lines, strategically placed hose lines will reduce water hammering by providing some accumulator effect.

• Avoid heavy pressure drops across components. Incorrectly designed components will cause cavitation where pressure is below the vapor pressure due to the low viscosity of water.

• Due to reduced compressibility, pressure peaks caused by braking inertia masses will be relatively larger than those of oil hydraulics. Subsequently, components must be designed for increased forces.

To speak with a Danfoss applications engineer in the U.S., call 414-884-7400.
To speak to a Danfoss applications engineer in Denmark, call 45 74 88 52 72.