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Total cost of ownership of linear systems
There are many factors to consider when looking to purchase a linear system, and invariably cost is one of the fundamental ones. However, using the purchase cost alone for economic guidance can be very misleading. The real cost of the product must be considered based on installation costs, maintenance costs, downtime costs and replacement costs, along side the purchase price.
The standard choice for many designers based on purchase price alone is likely to be ball rails, and in many situations this type of system will meet the demand of the application. However, depending on the environmental conditions, required system life and hourly downtime costs, engineers may be surprised at how the total cost of ownership (TCO) can rise. Conversely, alternatives such as V-based systems often have a higher purchase price, but can actually end up costing less.
The cost savings for V-based linear guides become apparent early on, even before the machine is running. Optimum performance of any guidance system will always be achieved when the surface that it is mounted to is flat, and if rails in parallel are set correctly. The inherent high rigidity of ball rail systems means they take longer to install, owing to the need for precision mounting surfaces and relative parallel setting.
Typical figures taken from manufacturers technical data would state that a rail parallelism tolerance of 0.030mm over the entire travel length for a standard-sized bearing (size 25 block) will be required with normal block clearance. Any binding or increased friction from setting inaccuracies is likely to lead to a reduction in system life. The setting procedure can be particularly demanding for ball rails over long lengths, due to the additional requirement of achieving smooth running of both parallel guides. For optimum life and performance, ball rail systems should be mounted to surfaces that have been machined flat prior to installation.
Considering the time required to ensure a smooth running ball rail system, products such as HepcoMotion’s GV3 linear guides have found favour with many designers who want to be able to fit a system to a surface with minimal preparatory work. Hepco’s GV3 system is a
V-based linear guide designed to serve a diverse range of automation and linear applications. Hepco’s V guide system functions on the basis of tight geometrical control over matching slide and bearing Vs. In turn this provides a wiping action that expels debris, keeping machines running and reliable. This V guide system has a high level of built-in compliance that will enable consistent running along the whole length, even when the mounting surfaces are not exactly flat.
Options for parallel setting include GV3 floating bearings that allow a small amount of axial movement. For example, Hepco floating bearings allow a V parallel misalignment of up to 2.0mm of the opposing V system. V guides can also be used with standard flat track and rollers providing even greater axial movement. With the emphasis on parallelism reduced, it’s clear to see how engineers can save valuable hours by using linear guide systems such as Hepco’s V guide. These savings are particularly noticeable for large production lines where multi-machine movements will involve many linear systems.
Whilst the quick mounting design provides clear savings on installation costs, V guides really come into their own when assembled and production commences. The packaging industry, for instance, is notoriously fast-paced, with demanding applications, making it a good example to demonstrate the potential cost-savings. Here, many of the machine movements will be working at speeds in excess of 2m/s and possibly subject to short stroke reciprocating movements. This could mean that over the period of a year, the majority cover a distance of 25,000km or even further.
Packaging applications are highly demanding, and an expected machine life of over ten years is typical. In a relatively clean environment, it can be expected that linear systems within a typical packaging line will need replacing at least once during this period. When replacing a ball rail system, both the block and rail must be discarded even if the rail shows no sign of wear. This is because the block and rail are closely matched together, and cannot tolerate standard replacement component parts once the system has been worn in. Throwing away the entire system when only part of it is worn is not only expensive, but very wasteful and environmentally unfriendly.
When a Hepco V guide system shows signs of wear, the eccentric adjustment facility of the V bearings can be used to quickly and simply remove any play that has occurred. When the V bearings reach the end of their calculated life, they can be replaced individually, and not as an entire set, so perfectly good components do not have to be discarded unnecessarily.
Typical stroke length applications will see the linear rail last approximately ten times the life of the V bearings or ball rail block. This is because the bearings or block are constantly in contact and under load, while any given section of the rail will be loaded intermittently in comparison (except for very short stroke length applications). Hepco's V guide system allows for individual bearings to be adjusted and then replaced as required, but the V rail can stay in service throughout this process and until it has reached the end of its natural life expectancy. This principle is not only economic and cost efficient, but far less wasteful than ball rail systems.
Downtime per hour, per system differs dramatically between industries and specific manufacturers. Customers within the packaging industry have downtime costs varying from £500 to £100,000 per hour, highlighting how longer downtimes can really impact on businesses. Although ball rails are cheaper to purchase initially, in the long run, for some applications, they can actually end up costing more. For example, when compared to replacing an entire ball rail system (block and rail), the replacement of individual Hepco V bearings is a much quicker process. It will take approximately 30 minutes to change each ball rail system (including disassembly and setting work), while four bearings can be changed on an average GV3 carriage in less than ten minutes. This time difference has an impact on not only the hourly costs, but also on the downtime costs, making the Hepco system far more attractive from a maintenance perspective.
Of course, environmental conditions will also play a part when considering the TCO. Hepco’s V guide system has an inherent wiping action due to the geometry of the slide and bearing interface. This means that dust, dirt and debris is expelled from the running surface and the system works perfectly well in contaminated environments. Conversely in a ball rail system, if particles of dirt and debris find their way beyond the seals, or if they become damaged, the internal ball bearing track will become jammed and motion will be impeded. This can result in catastrophic failure – meaning excessive downtime, and a complete system change.
Hepco's V guide system not only thrives in harsh environments, it also keeps on working after bearings come to the end of their lives and does not fail catastrophically. In many cases, customers do not even notice that an individual V bearing has failed until planned maintenance and inspection.
Ball rails have become the default choice in many installations and are proven to be a reliable product in the vast majority of cases. However, a greater examination of Hepco’s GV3 product does reveal some key and important benefits. This can be seen particularly in longer length applications where setting of profile rails can be time consuming compared to V guides that offer more effective solutions. Another key factor is the requirement for systems to work in hostile conditions; V guides have been proven to be a highly effective solution under such conditions.
Whilst purchase cost is always a key factor, it clearly pays to look into the system mounting/installation options, together with the longer-term maintenance issues. To this end GV3 is often found to be an easy to assemble reliable performer that offers a reduced engineering package not often seen with other linear solutions.
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