Protecting drive chain systems - what are the options?
Drive chain systems are used in a vast array of applications, from the largest machines built by man - such as open mine bucket excavators - to the humble printer; and they all need some form of protection device to prevent overloading of the drive chains. However, there are a large number of options with various mechanical and electronic devices available.
Greg Sharpe, Engineering Manager for Tsubaki Europe, provides some insight.
For anyone involved in specifying chain protection systems, the first objective is to understand the application and appreciate the specific requirements that the protection system must fulfil. This information allows an informed decision to be made on the complexity and cost of the protection system which will be required to prevent costly downtime and minimise maintenance costs.
The most basic and cost effective mechanical units use friction plates where the pressure applied to them is manually adjusted. While these are very simple to install, they have to be reset manually, and the friction plates are wearing parts that may need to be replaced periodically. For processes with low asset value where precise positioning and reset times are not critical, the friction plate torque limiter is an ideal solution.
However, for many applications a different approach may be required; one that can be adapted to suit different environments and processes. The basic principle involves a non-symmetric arrangement of ball bearings located in small pockets being held in place by a spring loaded plate. In an overload situation the ball bearings are dislodged from the pockets and the drive is disengaged.
This design principle offers many advantages especially with regard to the accuracy of the torque trip point. Even with repeated trips, the torque setting should remain within a tolerance of ±10% for a general purpose device. Once the cause of the overload has been removed, the protection device will automatically re-engage when the drive side is rotated.
These devices can be integrated into the process control structure with the addition of proximity sensors which allow an overload event to be detected. Once detected the signal can be used to stop the drive motor and trigger an alarm. This reduces the possibility of any danger from the overload situation and can improve the response time by the maintenance engineers, who need to clear the cause of the overload and reset the drive motor controls.
The basic design principle can be refined to provide zero backlash for applications that require highly accurate positioning. For applications where the working environment is contaminated with dust, oil or water, it is possible to produce completely sealed units that can perform in arduous circumstances without intervention by the maintenance engineers.
As safety devices, these products will be most effective if they are installed in the place nearest to where the overload is most likely to occur on the driven machine. In this way only the equipment immediately affected by the jam or overload is stopped. However, in some applications where additional equipment needs to be halted to avoid making the situation worse, this can be achieved using electrical signals from the torque protection device.
An alternative approach is the use of electrical relays to detect overload situations by sensing the motor current and using the relay contacts to initiate a shutdown of the motor in question. These devices are easily installed in the existing process control panel, and linked into the required circuits to ensure a safe stoppage in the event of an overload.
This should not be confused with the thermal overload relays that protect the motor from burnout. The current sensing overload relay can be set up to ensure that small peaks in current are ignored, reducing nuisance tripping but ensuring that genuine overload situations case the relay to trip. The simplicity of these relays makes them ideal for installation on existing equipment that is not fitted with a mechanical protection device.
Tsubaki, one of the world's leading manufacturers of premium quality chain products has also developed a class leading range of mechanical drive protection devices that covers a wide range of applications. The safety and control range includes Torque Limiters and Torque Guards as well as electronic protection systems such as Shock Relays and Shock Monitors that can be set up to provide vital protection and control to existing equipment.