A decade ago, automobile manufacturers introduced cars with anti-lock brake systems (ABS) that were controlled with the help of speed sensors added to the wheel bearings. Now bearing manufacturers have set their sights on sensing applications such as speed detection for off-highway mobile equipment. In other cases, the technology is reaching into new areas for industrial applications, detecting parameters other than speed.
Progress on the industrial side is slow however, with only a few smart bearings now available. But expect more to come in the next few years as engineers work out the design kinks.
ABS kicks it off
Speed sensors were first connected to wheel bearings for use with anti-lock brake systems (ABS) in the late 1980s. In most of these systems, the sensors detect pulses from a ring mounted on the outer part of the wheel bearing. Each sensor counts wheel revolutions and provides feedback for electronic control of brake fluid pressure according to the type of road surface.
With the early systems, the sensors were mounted near the tires, exposing them to water, dirt, stones, and chemicals. But newer systems have integrated the sensor into the bearing to protect it from exposure to the outdoor elements.
For example, the Sensor-Pac bearing from The Timken Co., Canton, Ohio, contains an integrated sensor that is protected from hazards such as water and dirt, as well as thermal shock and electromagnetic interference. The Hall-effect sensor fits within the bearing without increasing its overall dimensions.
This bolt-on smart bearing detects wheel speed and produces a signal proportional to speed, which helps controllers improve performance of anti-lock brake systems (ABS) and traction control systems (TCS). Its speed information can also be used for vehicle dynamics control, navigation systems, and speed control. It uses a micro-chip speed-sensing element that can sense down to zero speed.
The ABS Sensor Integrated Hub from NTN Bearing Corp., Mt. Prospect, Ill., contains a pulse ring and sensor. The pulse ring is a permanent magnet with many pairs of north and south poles. Because both pulse ring and sensor are located inside the hub bearing, there is little risk of damage from water and dirt.
SKF wheel hub bearings use a built-in sensor to detect speed, position, and direction of rotation in ABS brakes. These sensors are applied to deep groove ball bearings. Other units are used on automotive fuel pumps and active suspension systems.
The sensing unit is integrally mounted on the side of a bearing and requires a few millimeters more width to the bearing. It is not available separately for retrofit mounting to an existing bearing.
Active Sensor Bearing (ASB) technology from The Torrington Co., Torrington, Conn., integrates bearing and sensor functions to simplify system design and reduce assembly cost. Sensor assembly is by a clip-on method. Torrington is offering licensing and technical support for ASB to bearing manufacturers.
This bearing incorporates a magnetic encoder whose rotating position is detected by an external sensor. It provides a digital output signal proportional to wheel speeds down to zero.
Off the road
Smart bearing technology started to move into the industrial domain in the mid 1990s. Not surprisingly, initial efforts have focused on measuring speed in mobile equipment.
For example, Timken is investigating the possibility of smart bearings in two major areas: off-highway equipment such as backhoes, earth moving equipment, and railroad cars, plus machines used in industrial plants.
But in addition to speed, such devices are expected to measure parameters such as vibration, temperature, and load – using accelerometers, thermocouples, and strain gages respectively. Bearings in off-highway equipment are generally large enough so there is plenty of space to add multiple sensors for detecting several parameters.
These devices will be linked to computers in the operator's cab. Such computers can analyze the measured data and indicate corrective action, if any. They can even be programmed to shut down the equipment.
NTN engineers are developing speed-sensing bearings for use in large off-highway construction and agricultural equipment. In a dump truck, for example, such a bearing could sense wheel speed for automatic differential locking, a function that is now done manually.
Meanwhile, SKF is expanding its automotive applications to include active suspension systems and fuel pumps in addition to wheel hub units. Other potential applications include fork lift trucks, traction motors, and transmission gearboxes. Smart bearings for fork lift trucks can sense wheel position for use in the steering system, and detect fork lift position (height) to help place loads on racks.
Industrial machines
Increasing diagnostic capabilities of built-in sensors can also benefit rotating bearings on industrial equipment. One example is the EZlink monitoring system developed by Rockwell Automation/Dodge, Greenville, S.C. In this system, sensors embedded in the bearing housings measure bearing temperature, vibration, or speed. A DeviceNet bus transmits sensor data from up to 62 bearings to an industrial control, such as a PLC. DeviceNet eliminates the need for discrete power supplies, letting the sensors take power from the network. And it requires less wiring than traditional methods. Alternatively, data can be sent to a PC using an RS 232 DeviceNet converter. The EZLink system is also used to monitor geared speed reducers.
For standalone systems, engineers have two options for monitoring operating temperature. One is a lowcost temperature switch embedded in the bearing housing. Normally open, it closes at a predetermined temperature (between 150 and 260 F) to initiate a warning.
The second is an embedded thermocouple that connects to a transmitter in the bearing pillow block. The transmitter mounts close to the thermocouple to avoid interference or signal problems. It uses a 4-20 mA signal, which can be scaled to indicate temperatures from 0 F (4 mA) to 250 F (20 mA). The thermocouple detects gradual temperature changes, such as might occur due to insufficient lubricant, and signals the need for predictive maintenance.
To measure vibration, the system uses a piezoelectric accelerometer that detects vibration amplitude (up to 50 g) and frequency. All components mechanically connected to the bearing are part of the vibration signature. This lets you monitor changes throughout the drive, such as wear in a shaft, bushing, or pulley as well as the bearing. The signal goes to a vibration transmitter that produces a 4-20 mA signal proportional to the vibration velocity or a 65 V signal proportional to acceleration, and sends it to a PLC or PC.
An ac or dc proximity sensor on the bearing pillow block will detect speeds to 600 rpm by measuring revolutions of an adjacent locknut that has two metal sensing targets. This method is used on tail pulley fixed bearings to detect slippage or stopping of a conveyor belt. The bearing sensor can connect to a PLC as long as program timing is adjusted so it can capture all the pulses.
Sensor output for the various parameters can go to a PLC scanner for comparison with programmed limits and for action when they are exceeded. Or it can be used to monitor speed, temperature, or vibration and to develop histogram plots of these variables over time.
The sensors are available on pillow block bearing housings of ball, spherical roller, and tapered roller bearings with shaft sizes of 27/16-in. to 16 in.