Slip Rings- Slew Indication

Posted on Tuesday, August 10, 2010 in UEA Blog

An electric slip ring assembly is used to conduct electrical power and signals throughout a continuous 360 degree rotation.  In some applications it is also important to have some means of indicating areas or specific positions within the 360 degrees.   This can be done in a variety of ways depending upon the actual number of positions or areas required, whether they are permanently set or need to be programmable and upon the accuracy required.   For the simplest applications, such as a single 'home' position on a small crane, amusement ride or indexing table, UEA has for approximately 30 years incorporated a laser cut cam plate into the center core stack of rings and insulators in the slip rings.  As the slip ring rotates the roller on a spring-loaded arm follows the contours of the cam and actuates a small single-pole, double –throw switch.   An accuracy of plus or minus one degree is achievable and repeatable with this design with up to three areas defined over the 360 degree rotation with one cam.   Another cam can be added for additional defined areas but obviously adds additional height to the core stack.

For more than 15 years UEA has offered the option of a continuous rotation potentiometer that supplies a unique signal for any point during the 360 degree rotation of the equipment to which it is attached.  Normally supplied with a slip ring assembly, the potentiometers provide an analog signal that is converted by customer supplied software, such as that used for a crane LMI (Load Moment Indicator) system, into a given position.  The LMI system uses the rotational position, along with boom angle, boom extension and the weight of the part being lifted to compare to the lift charts having already been determined for that crane.  Because the outriggers extended to support a crane during a lift operation are not normally square in pattern, a typical crane can lift more over the front and rear of the machine than over the sides and the lift charts reflect that variation.  The potentiometers are a vital part of the LMI safety system for lifting but they also provide another safety feature that is not available with the cam and switch system.  Using the potentiometer position signal, the crane boom can be swung to a position and a stop or set point established.  Another set point can then be established which will then limit the crane to movement between these two points.  This is particularly important if the crane is working close to buildings, power lines, etc.  By establishing swing limits the crane operator is automatically warned as the boom swings close to the danger points.  UEA has two potentiometer styles with one being a traditional 2.00” O.D. with shaft drive and the other being  4.00” O.D. with a 1.50” through bore for use with slip rings that need to pass pneumatic, hydraulic or other types of passages through the assembly.

A variety of encoders have also been used with UEA slip ring assemblies over the years but relatively new on the scene are encoders with a CANopen or J1939 output.  These are most frequently used, at this point, on mobile or construction equipment with similar needs to those supplied by the potentiometers.  Because they operate on a CAN communications system the encoder can become just another sensor on the system and its position above or below the center of rotation is immaterial once attached to the CANbus circuits passing through the slip ring.   As more engine controls and remote sensors adopt the CANopen or J1939 output the use of this type of encoder will become even more popular.

As always, United Equipment Accessories, Inc. is available to answer any questions you may have about our products, or this blog post.  Please feel free to contact us at 1-877-352-4739 or email at We promise that you will speak directly to an employee in our office, never an answering service!  Check out for more helpful information about United Equipment Accessories Inc, and our products.  Hope to hear from you soon!

-Brent, Senior Design Engineer