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Do you have problems with robotic grippers?

29 August 2012

Some food manufacturers run into problems when their robotic grippers start to wear. FP Express asked industry experts how processors can avoid these problems.

Last month, our main case study in the print edition of Food Processing featured a problem that Unilever faced when its Twister production line ran into gripper complications during pick-and-place operations. The pick-and-place gripper moves across and distributes the ice creams onto an eight-lane conveyor system, from where the product goes on to packing.

But the grippers being used were showing signs of wear and tear; in addition, they were failing to hold properly onto the ice creams and were allowing them to fall onto the production floor. Apart from being inefficient, this was obviously unacceptable.

And, as Chris points out, the real problem was the unreliability of the gripper cylinders; each one had a working life of between only three to four weeks and was too costly to replace. Unilever estimates the annual cost for the replacement of the 180 cylinders was about £16,000 before labour costs.

Bosch Rexroth resolved the problem for Unilever. Steve Morris, an expert in pick-and-place technology for food applications at Bosch Rexroth said: "The gripper that had been specified was wrong for the job. The Twister line was equipped with a 180 degree gripper from open to close, which was at least two sizes beyond what was needed."

We decided to speak to various industry experts about problems with grippers, and whether there was a way food manufacturers could avoid choosing the wrong grippers in the first place. The responses to our question were illuminating.

"Robotics grippers are an integral part of the robotic arms used in a variety of food processing and packaging applications," says Nigel Smith, managing director of TM Robotics. "In the case of pneumatic robots where the grippers need to be sanitised with a high-pressure jet wash, the danger is that grippers can oxidise and rust. In the case of unpacked ingredients, this can present a contamination problem.

However, the biggest difficulty that comes with the wear and tear of grippers is that once they age, metal splinters can break off and fall into foodstuffs. "Robot manufacturers and integrators are always searching to manufacture grippers with higher quality non-metallic materials so as to make them stronger and reduce the risk of deterioration," Neil explains.

The suckers on vacuum grippers that handle delicate products such as eggs, confectionery or soft fruit, can wear out and lose their suction. In this case, they can leave a mark and damage the product they are supposed to pick and place safely.

"Ultimately, given that food safety is the main concern for manufacturers, provisions for robust grippers must be made. Payback times are a factor worth considering by all purchasing managers. Although better and more efficient grippers cost more, they will not wear out as quickly, alleviating the costs of maintenance, downtime and frequent purchasing of spares."

Garry Powell, operations director at Endoline Machinery agrees that the wearing of grippers can be a problem in the food and beverage industry.

"First, the contact faces of grippers are typically fronted with rubber or polymer of varying hardness for a high friction grip," says Gary. "High friction faces, unlike hardened faces, allow lower grip pressures so there is less chance of marking or damaging the product. As there are numerous gripper face materials of varying hardness and resilience, selecting the correct material may be crucial for good performance. Mechanical gripper mechanisms can wear if they've been poorly chosen; for example working at the limit of the gripper capacity or if they've been poorly aligned. The experience of an automation supplier with extensive application knowledge helps avoid premature wear or failure."

Richard Sawford, product manager for robotics at Marel says that complex robotic grippers in the food industry typically operate at up to 100 times a minute for 16 hours a day and are, therefore, subject to a great deal of wear.

"Being manufactured from the latest high-tech plastics renders them, in the main, immune from the effects of modern cleaning regimes,'' says Richard. "However, this intensive action can also remove essential lubricants with equal efficiency making it vital for components such as bearings to be able to withstand very high rates of wear and provide a long working life.

"Manufacturers of modern bearing materials can make many components such as bearing bushes on a bespoke basis and at a reasonable cost, making it easy and affordable to replace them at planned maintenance intervals, along with any pneumatic cylinders. To ensure continuous operation and working life, the provision of a second gripper cancels out any production downtime caused by routine maintenance or cleaning."

A different point of view is provided by Alan Spreckley, ABB segment manager for consumer industries and packaging. "In my experience, the wearing down of robotic grippers isn't an inherent problem in the food and beverage industries," notes Alan. "As demand for robotic automation has grown, leading manufacturers, including ABB and its partners, have invested in R&D to develop specific solutions, such as robotic grippers dedicated to an application. Consequently, a bigger issue today is the selection of the correct robotic grippers for the process or application in question."

Food producers and their chosen suppliers need to look at the most suitable equipment for their application and consider what the grippers will be used for, Alan advises. "There is a wide variety of grippers available, including proven solutions such as vacuum or mechanical grippers for handling products typically in secondary packaging or palletising applications. Also available are claws for high speed bag and sack palletising and even a range of specially designed grippers that have been developed specifically for use in high risk areas.

"These grippers are typically IP67 or IP69K and can be vacuum, mechanical, Benoulli type (contact free or minimum contact) or even a combination. With such options, investing in the right equipment is an absolute necessity. Once the right piece of equipment has been chosen, general wear and tear on the production line will be reduced, while productivity and efficiency will improve."

Paul Wilkinson, business development manager at Pacepacker Services says that although natural wear and tear can be expected over time, a good robotic supplier will provide recommended spare parts which are relatively low in cost, easy to use and require very little downtime. "As the demand for robotics has increased so too has the design and investment in developing robots and in particular the grippers,'' says Paul.

"Multiple grippers, or end effectors, are now widely accessible to suit a range of robotic applications - from suction cups to mechanical attachments - so food suppliers need to work with their chosen robotic supplier to ensure that they have the correct gripper to suit their particular robotic application.

"To reach this conclusion it is essential the product being picked by the robot is tested correctly with the chosen gripper - the robotic supplier's knowledge of applications should also come into effect during this process. Utilising the correct end effecter will enhance the efficiency of the production line and the risk of wearing down the grippers is reduced."

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