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How are slip rings used in crane

Introduction

For power and data transmission, the crane construction industry uses a heavy rotary application such as slip rings. Slip ring is a device that transmits electrical signals, power, and data between a stationary component and a moving component. 

Source – wiki

There are several other industries like towers, windmills, turbines, packaging machinery, etc. also use  slip rings. This duty type rated motors develop high starting torque and low starting current, which is suitable for a frequent start, stop and reversal. The reliability of the slip rings plays a vital role in providing smooth and efficient performance, especially with heavy loads. 

What is a slip ring, and how does it work?

An electromechanical device mainly helps in electrical and power signal transmission from a motionless structure to a structure which is rotating. It is almost used in a system that is electromechanical and requires rotation to transmit signals or power. 

It helps to ease system operation, improve performance, etc. Slip rings are also popular as collectors, swivel, rotary joints and rotary electrical interfaces.

As we know that slip rings to transfer power and data between two components. But how does a slip ring work?

Mainly it is made of two components – a metal ring and a brush contact. 

It works by holding a sliding brush contact against the metal ring. In the process, the brush part remains stationary, and the metal part rotates. The lead wires of the brush and conductor terminate internally and provide an electrical connection to the device on either side of the slip ring. 

The brushes are kept secured by the brush holder that mounts over the housing. A set of ball bearing inside the housing supports the rotor. The other components of the motor, like rotor, cover, and housing, are made of metallic and non-metallic materials. 

The selection of material mostly depends on temperature, power, weight, size, and cost. The electrical components usually have highly conductive materials due to various reasons, such as voltage, current density, resistance variations, rotation speed, bandwidth, etc.

Composition of slip rings

Created from a range of conductive materials, but mostly copper or gold alloy brush is used. The insulation that is used between the shaft and ring includes a wide range of synthetics. It also consists of metal contacts or graphite, which hits a rotating metal ring on the outer side diameter. 

Additionally, whenever there is a need for an electrical circuit (more than one), then it is required to arrange the rotating axis along with the brush gatherings. The rings or brushes remain still, whereas the other components keep rotating. 

Types of slip rings

  1. Wireless slip rings

The wireless slip ring does not depend on the typical metal and carbon brush contact method. Instead, they transfer data and power wirelessly through an electromagnetic field, which are created by the coils that are placed in the receiver and the transmitter. 

It is the upgraded form of the traditional slip ring. These slip rings can operate in harsh environments for a longer time and require less maintenance. However, it transmits a limited amount of power, where a traditional contact can transfer more power in the same volume.

  1. Pancake slip rings

In pancake slip rings, the conductors are placed on a flat disc as a concentric ring in the interior of a rotation shaft. The composition has a higher weight, volume, capacity and brush wear. 

It collects the brush wear debris on the vertical axis. It offers a reduced axial length for the number of circuits, which is appropriate for some applications. 

Image – Pancake slip Ring

Source – Moflon.com

  1. Mercury wetted slip rings

These are well known for their stable connection and low resistance. It uses a different process, where the brush contact is replaced by liquid metals that are bonded molecularly to the contacts. During the process, the molten metal tends to maintain the electrical connection amongst the rotating contacts and stationary contacts.

However, using a mercury-wetted ring is dangerous as it contains a toxic element. If the application involves in a food manufacturing process or pharmaceutical equipment, then the choice has to be made carefully while choosing metal contacts.

Common uses of slip rings in crane technology

Image 2 – Slip Ring for cranes

Source – directindustry

Slip rings are widely used in crane technology for power and data transmission. In heavy-duty conditions, the rotating components of the crane must be reliable, powerful and must supply data in indoor and outdoor applications. Mostly gold wire technology of Rotar X slip rings is used in cranes. It ensures durability and trouble-free operation of the system.

The common uses of slip rings in crane technology

  • The crane slip ring offers maximum flexibility for a wide range of applications.
  • Slip rings produce high torque with the low starting current.
  • Crane slip ring assembly ensures effective and efficient operation even in a harsh climate. 
  • The speed of the rotation can be managed and adjusted easily with the help of a slip ring.
  • A squirrel cage induction motor needs 600 % to 700 % of the full load current, whereas, a slip ring induction motor needs 250 % to 350 % of the full load current.

The other typical applications of slip rings are

  • Mobile cranes
  • Excavators in the building industry
  • Tower cranes
  • Bucket-wheel excavator in a mining
  • Cable reels for harbor cranes
  • Rotating superstructures for fire engines
  • Pillar jib cranes

Slip rings for power transmission in cranes

Crane duty motors are suitable for three-phase supply. Generally, the use of power does not affect the slip ring’s ability. But the voltage drop changes the actual voltage available at the load, and the total power gets dissolved in the slip ring. 

The dissolved power in the slip ring converts to heat and can agitate the operating temperature. Luckily, the components in the slip rings do not overheat to pose the risk of burning.

Conclusion

Slip rings are for any type of application that deals with a rotating base or platform. It is extensively used in AC generators and DC motors. Although the traditional use of a slip ring is to transmit power, they can also send analog and digital signals from devices. It can even transfer data. 

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