Roberto Ferrero
Global Applications Specialist
SKF Electrical Div.
Airasca, Italy

Edited by Sherri Koucky

Adding grease to wear-resistant rubber seals reinforced   with steel ensures a –40 to 120°C operating range.

Adding grease to wear-resistant rubber seals reinforced with steel ensures a –40 to 120°C operating range.


Every year, worldwide demand for washing machines, dryers, refrigerators, ranges, microwave ovens, dishwashers, vacuum cleaners, and air-conditioning units and fans, tops around half a billion units.

That's a lot of appliances. All of them could potentially wind up in landfills someday. Until then, they will soak up juice from power grids around the world. So it's no wonder that appliances are increasingly subject to government mandates for energy efficiency and recyclability. For example, the European Community and U.S. Federal Agencies (FTC) have established energyconsumption regulations. These regulations cover energy efficiency, noise levels, durability, and recyclability.

The quest to meet regulations on energy consumption has led to developments such as better thermal insulation of refrigerators and horizontal-axis washers. Interesting results in the latter area have come from the Thelma Project (The High-Efficiency Laundry Metering & Marketing Analysis) sponsored by several companies and government agencies. It found that H-axis washers bested older vertical-axis designs in areas that include energy consumption (45% less), average water consumption (25% at maximum fill level), and less energy needed to dry clothes, thanks to a faster spin cycle that removes more moisture.

A combination of electronic controls and new developments in drive systems make such advances possible. Motor technology in particular has gotten scrutiny. Though universal brush motors are still popular for

some domestic appliances such as vacuum cleaners, there is the need to find alternative motor technology that is more efficient and less noisy.

One candidate is the technology of splitphase induction motors. They are quiet (no brushes) and last a long time. But they have limitations such as high in-rush currents at starting, a speed that depends on the line frequency, and efficiency that is only moderate.

Within this category falls switched reluctance motors (SRM), brushless permanentmagnet motors (BPM), and controlled induction motors (CIM). The biggest advantage they offer is a design that lets appliance manufacturers tailor motor output to the final application.

In washing-machine motors, for example, these technologies let designers save energy in several ways. They can agitate laundry at different speeds depending on load size. Also possible is bidirectional washing action, customized wash/rinse cycles depending on the laundry load, and an ability to compensate for unbalanced loads. All these factors help out water and energy usage.

There is no question that these motor technologies still have limitations. Their physical makeup is complex and the electronics and software for their control can be quite involved, especially for SRM and CIM. Nevertheless, it remains a big area of development for efficient appliances.

To operate, SRM and BPM motors require some means of detecting rotor position. When used for washing-machine drives, speed detection is also a must to regulate all the different washing and rinsing programs. Tachometers mounted on the motor shaft usually supply the speed feedback. This approach is well known but implies elaborate assemblies of several components. These problems can now be solved by sensor-bearing units.

Sensor bearings come out of work on ABS, antilock braking systems. ABS sensors are today built into hub-unit wheel bearings. This gave rise to the integration of an active sensor with a deep-groove ball bearing, widely used in domestic appliances.

The bearing sensor is essentially an incremental encoder. It measures accurately down to zero speed, normally generating between 32 and 80 pulses/rev with a maximum resolution of up to 1.4 angular degrees.

The sensor body has two Hall-effect cells that sit on a single integrated circuit. The two cells are slightly offset from one another and allow other circuitry on the chip to determine the direction of rotation.

Bearing friction is also getting a lot of attention because it is a big source of energy loss. The friction in a rolling bearing depends on the load, bearing type and size, operating speed, and the properties of the lubricant and quantity. The total resistance to rolling in a bearing consists of the friction in the rolling contacts, in the contact areas between rolling elements and cage, and in the lubricant. In the case of sealed bearings, there is also sliding friction of rubbing seals.

The load-dependent frictional moment arises from elastic deformations and partial sliding in the contacts between rolling elements and ring raceways. The magnitude of the load carried by the individual rolling element depends on the internal geometry of the bearing, the bearing type, and the type of load applied to the bearing. Besides seeing forces from applied loads, rolling elements are also subjected to forces caused by speed effects. Typical of these are centrifugal forces, gyroscopic moments, and inertial forces.

Recent work has put a priority on cutting friction losses. In one case, engineers developing vacuum cleaners devised special ballsets able to hit 60,000 rpm for a 608 basic bearing size.

A cage keeps the rolling elements at an appropriate distance from each other. This distributes the load and prevents immediate contact between two neighboring rolling elements. This minimizes friction and relative heat generation in the bearing. A key part of the design is the use of cages molded of heat-stabilized, glass-fiber-reinforced polyamide 6.6.

This material combines strength and elasticity. Low friction arises from the sliding properties of the plastic together with the smoothness of the cage surfaces touching the rolling elements. Also, the low density of the material means the inertia of the cage is small.

Polyamide cages have excellent running properties under lubricant starvation. This property could potentially lengthen service life so long as the bearing stays within its operating temperature range (–40°C to 120°C).

Polyamide cages below –40°C lose elasticity, but can tolerate brief periods at up to 20°C above the recommended maximum temperature. This assumes, though, a sufficiently long cooling-off period and that the lubricant doesn't exceed its recommended temperature.

The type of lubricant can have a particularly large impact on friction torque in highspeed and light-load applications. The process of specifying a low-friction grease can be complicated. The type used can depend on noise level required, speed, temperature range, load-carrying characteristics, and rust-inhibition properties. Consequently, major bearing manufacturers offer engineering advice to help in selecting the best grease.

The amount of grease also impacts friction torque. Deep-groove ball bearings generally carry enough grease to fill 25 to 35% of the free internal volume. Less could be used if conditions permit it.

Deep groove-ball bearings sometimes carry metallic shields or rubber seals to cut noise or keep out dirt. The choice of capping material depends on factors such as speed at the sealing surface, the friction in the seal, the tightness of the seal against external contamination, the shaft arrangement (vertical or horizontal), and operating temperature.

Friction torque and sealing efficiency are interrelated. For sheet-metal shields, friction torque is not an issue because nothing touches the surface. But there is friction torque with rubber seals that comes from the seal rubbing action. This, plus friction torque from the ring surface, could greatly exceed that from the grease, cage, or load-dependent friction.

Low-noise requirements
Air conditioners, dishwashers, fans, and other motor-driven appliances have gotten close scrutiny for noise pollution. Noise in this context means airborne noise. It results from vibrations induced and amplified by the structure and comes from different sources. Among them are electromagnetic disturbances, fans, collectors, brushes and, of course, bearings.

In case of bearings, the root cause is essentially the contact between rolling elements and raceways, which is a function of internal clearance and preload. Precision, surface finish, quality, and characteristics of the bearing components (rolling elements, rings, cage, and grease) also play vital roles in vibration and sound generation.

Special frequency-analysis equipment can measure and pinpoint the nature of these vibrations. This equipment can, for instance, determine the effect bearing quality has on noise, or suggest the best types of cage or grease to ensure quiet operation.

These aspects are now essential parts of the design and manufacturing process. Bearing manufacturers can offer extensive application engineering knowledge.

Dedicated tools and software can also analyze areas where bearings mount. This can precisely identify if noise from an electric motor is magnetic based, coming from imbalances or misalignments of housings and shaft or, in the case of bearings damaged from improper mounting, it is due to inner ring, outer ring, or ball.

Durability
Governmental regulations may soon mandate a minimum life for appliances. For example, one proposal would require appliance manufacturers to guarantee that washing machines function for at least two years from the date of delivery.

For this reason, bearing selection must be based on quality and performance, as well as cost, in order not to jeopardize the service life of the appliance.

Involving bearing manufacturers in the design process helps balance cost and service life by tailoring bearing parameters to performance and life expectations. These include:
— Special ring materials.
— A combination of efficient sealing, polyamide cage, and special grease with EP additives in case of heavy-load applications, such as washers.
— Special shields, tailored internal design, polyamide cages, and high-speed, low-friction grease for high-speed dry vacuum cleaners.

Easy disassembly of the machine at the end of the life cycle must be taken into account in the design phase as well. In this regard, integrating functions such as speed sensing in bearings could be a valuable help.