Industrial Automation Magazine India

Tips to Reduce MTTR

Published on : Wednesday 02-09-2020

Shacheendra Bapat offers a few tips on how to reduce Mean Time To Repair even before a machine is brought under production.

Every organisation has a team that selects a new machine or equipment. This team generally is called as Manufacturing Engineering (ME) team. Its main responsibility is to select the most appropriate machine that is not only the most appropriate for the given process, but also the most cost effective as well. However, while these two aspects are always taken into account, or at least they should be taken, a very important aspect that is generally missed, is the maintainability of the machine or equipment post installation. The problem becomes more profound if the ME and Maintenance teams do not work in tandem. While the ME team works with the machine for a very limited period, the machine or the set of machines becomes a permanent baby of the Maintenance team for the rest of its life. While all the managements expect zero breakdown in their machines or at least very high Mean Time Between Failures (MTBF), the Maintenance teams thrive to reduce the Mean Time To Repair (MTTR). To achieve this, the Maintenance teams must be given the appropriate right to give their opinions and the ME teams must include them in their selection and design process for a machine. Here are certain tips that help to achieve the reduction in Mean Time To Repair and saving cost during the selection or design stage of the machine or equipment:

1. Every modern equipment by design, has a control system that includes PLC, VFD and other Input/Output elements. The new machines must have same or similar models and makes of these control system elements, which are already in use in a plant. This not only reduces the inventory cost (because the spares become common), it also eliminates the cost of procuring new programming software. If a specific model and make of a PLC is selected which is already in use, then the Maintenance team does not require to undertake training on other programming software. This also reduces the cost. Many organisations restrict the number of registered vendors, which is a good practice. If the control system components are standardised, then there is no need of registering new vendors. Since the Maintenance team develops expertise on the same model and make, it greatly helps in reducing the MTTR.

2. A machine, once designed, has a specific number of Inputs and Outputs. The PLC is then selected with a suitable configuration that can cater to these numbers. However, if the machine undergoes some minor design changes or some new variants are added to be manufactured on the same machine in future, then some Inputs or Outputs are needed to be added in the electrical circuit. In the wake of this, it is always useful to keep 5 or 6 inputs and output channels on the PLC as spare during the design stage itself. This not only helps in adding a new variant without the requirement of adding any hardware, it also reduces the MTTR in a specific way. For example, if a certain output is important for a process and some outputs are unused, then the free output address can be programmed in the PLC ladder and kept open without wiring. If that Output channel, which is connected to an Output element, fails, then it remains just the matter of removing the wire from that failed Output channel and connecting it to the Output terminal that was left open but is already programmed in the PLC Ladder.

3. The Control circuit of a machine invariably uses 24 Volt DC supply. There should be enough number of MCBs installed at the output of the 24 Volt DC supply that is specific to a given section of the circuit. There should be separate MCBs for Input wiring, Output wiring and wiring for annunciations. By adopting this method, the tripping of an MCB in a specific circuit does not cause failure of another circuit. For example, if there is a short circuit in the wiring of an output, then it will cause the MCB to trip. If the same MCB is connected to the Input wiring, the PLC will fail to register any other input that may cause unwanted movement of the machine. A separate wire can be connected between the outgoing terminal of an MCB and an Input channel of the PLC. A suitably programmed HMI message can be triggered to identify which MCB is tripped, in case one trips. This helps in identifying that exact part of the circuit which has a potential short circuit and this reduces the MTTR.

4. In most of the cases, the designer of PLC Ladder and the user of the PLC (the Maintenance Team) are different. It is the responsibility of the designer to make the PLC Ladder as decipherable as possible. I have seen PLC ladders which are very difficult to troubleshoot because they are not written in the way they should be. One reason behind this is that the designer wants to keep his one-upmanship intact and purposefully writes the ladder in a difficult way that could be read by him only. Such practice must be avoided and the ME team must advise the designer to write simple ladders. The Maintenance team must have a member who is good in writing and deciphering the PLC ladders himself. Having such person greatly reduces the MTTR. I have seen people struggling to identify simple problems like some specific feedback from an Input Element not being received by the PLC and the machine remaining under breakdown for a long time.

5. A device called Human Machine Interface (HMI) is a great tool that assists the Maintenance team in identifying potential or real problems. An appropriate HMI must be selected in the design of the control system from the initial stage itself.

6. Many machines have pneumatic or hydraulic cylinders for causing transverse movements. These cylinders, by default, must have two Input elements mounted on them, one for detecting the forward position and one for the retract position. Using only one input (its positive feedback for retract position and negative feedback for forward position) on a cylinder is not a good practice. MTTR can be reduced by using the feedbacks from these elements suitably. For

example, the transverse movement of a cylinder takes some finite time. It can be different for the two different movements. If a command is given by the PLC to a certain cylinder to move forward, then the cylinder forward position Input should become on within a specific time. If it does not become on then either the cylinder has not moved forward, or has not moved to its forward position completely or the Input element itself has failed. To identify the problem quickly, the PLC ladder can be written in such a way that after triggering an Output, if the intended input does not become on in a specific time, then a suitable message can be displayed on the HMI. This helps the maintenance team in identifying and resolving a problem very
quickly. The ME and Maintenance teams must ensure that the most correct text is selected for displaying the messages. A message “Hydraulic Motor OLR tripped” makes much more sense than the message “OLR 1 tripped”. A message “Fixture 1 did not retract” is far better than “Cylinder 1 did not retract”. Such messages are composed at the design stage itself and greatly help in reducing the MTTR. The text of the message must be simple and should make sense to the Maintenance Team.

7. Many big machines utilise the concept of networks, which have multiple nodes, to reduce the amount of wiring. Appropriate messages must be programmed in the HMI to notify the exact number and location of a node, if it happens to fail. Finding the location of a failed node installed in big machines, consumes a lot of time. So appropriately programmed messages help in reducing the MTTR.

8. Many machines have multiple motors installed for various process operations. A special tab can be programmed on the HMI home page, which when selected, will display the dynamic current being drawn by all the motors. Modern Variable Frequency Drives can communicate with the PLC through serial networks and the data of the amount of current being drawn can be made available to the PLC. Daily monitoring of current of these motors can be a good tool to predict potential failures in the Motors, their drive trains or the mechanics being driven by them without even using a clamp meter. While an increased current may not necessarily indicate a failure and stop the machine, a warning message can be triggered to take suitable action during the break time or off days.

9. A PLC becomes slow if its User Memory (where the PLC ladder program is stored) is consumed for over 90%. Appropriate size of the User Memory must be selected so as to keep its consumed memory to a level of about 75%. The program may need some additions based on the findings of a route cause analysis of a problem that had resulted in a breakdown.

10. A cramped up control panel may help in reducing the size of a machine or may have a reduced cost. However, the control components mounted closed to each other and having cramped cable treys cause significant problem in troubleshooting thereby increasing the MTTR. Enough space should be left around the components like PLC, VFD, SMPS and contactors to facilitate troubleshooting.

11. The Bill of Material for the Control System should have as less number of items as possible. It makes sense to use only M12 or only M8 size of Proximity switches on a given fixture, if the dimensions allow. It is better to modify the design of a proximity mounting bracket at the design stage itself so that only one size of proximity switches is required to be used.

Shacheendra Bapat, graduate in Electronics and Telecommunication engineering, is an Automotive and Manufacturing Industry professional having more than 26 years of experience in operation and Maintenance of Flexible Manufacturing System, HMCs, VMCs, SPMs, welding machines, paint shop equipment and conveyors, with several MNC and large Indian engineering companies. A Technical Trainer for Industrial Automation, Electrical Safe Work Practices, Maintenance Safety, Chemical Safety, Predictive and Preventive Maintenance, etc., Shacheendra is also a Certified Lead Auditor for QMS ISO 9001:2008, Certified Internal Auditor for EMS ISO 14001:2004 and Certified Project Management Associate IPMA Level D. He has worked with Hindustan Motors Ltd, Eicher Motors Ltd, General Motors Ltd and Bosch Rexroth India Ltd.