Despite extensive precautions, not every linear bearing will always function as expected. IKO lists some common issues with recently-installed linear bearings, and how it fixed them.
Performance issues with linear bearings are often related to what they are bolted into, explains the Business and Operations Manager. Guides, for example, are generally mounted to a bed, which should be a certain thickness; a minimum of 6mm plate or similar structural surface. If, however, the customer is bolting in to 3mm plate, doesn’t read the instructions on how to mount it, gets the holes pre-drilled, inserts the screws, tightens everything up and puts the carriage on, it may only get halfway down the rail until stops. The mistake is to have overtightened the bolts. Since the linear guide is the strongest part of the assembly, it is warping everything, so the carriage locks up. That is simple to resolve: loosen off the bolts. In this situation, customers should check the thickness of the plate, and consider if it is twisting.
Depending on the design of the linear guide, the mounting surface of the carriage is a U-shaped profile, adds the General Manager. As the bearing can take a huge load, customers tend to assume the structure is sound, he points out. But actually if it is mounted to a surface that is convex, then the profile will open. By the same token, if you mount it to something concave, the profile will close; in both cases, the carriage will lock. He adds: “We know the characteristics of the product and the regular problems that people come across, so we’re very quick to highlight those, and tell people to look at their mounting surface and tolerances. Sometimes simple conversations can alleviate these problems.”
On the subject of rail, the General Manager recalls a case where IKO had supplied linear guides running on long sections of rail whose joints were butt-ended flush, but after six months the ends had tilted up toward each other, and the carriages wouldn’t run. It transpired that where the machine was installed, summer temperatures were so high that the rails expanded and pushed themselves up – which was news to the IKO team, as the customer had not mentioned this during specification. Had it done so, IKO UK could have added, as a special design alteration, an expansion gap between rail ends. Such gaps could measure up to 2-3mm in linear ways with carriages fitted with a 6mm-diameter ball without affecting the running of the carriage.
The unfamiliar technology of linear bearings can also catch some customers out. As linear bearings incorporate circuits of rolling elements, they require special handling. In a recent conversation, the Business and Operations Manager recalled that a customer said: “We have this new guide from you, and it was running a bit funny, so we turned it over, and there’s some balls missing.” At this point he points out that it is not physically possible for the balls to come out when a carriage is on the rail. So when he asked, ‘Has anybody taken it off?’ the customer replied, ‘No, it came in like that.’ But that wasn’t the case. “What’s happened is, someone has opened that [carriage] up, and they’ve turned it, and the carriage has fallen off and the balls have come out. It happens all the time with people who haven’t handled guideways. So I said, ‘Send it back, I’ll have a look at measuring the balls and we’ll pop a few more balls in there and get it working.’”
Although not common, performance issues can also spark a change in specification. An IKO UK technical Sales Engineer recalls two instances where a customer requested a change in the degree of preload in the carriage. Preload is all to do with frictional resistance. The higher the preload, the higher the rigidity of the system, but also the greater the frictional resistance; the harder it is to move it. In one case, IKO supplied standard preload and the customer requested more, and in another case, the customer actually required greater clearance for freer movement – not that it knew it at the time.
That customer was making a machine that automated laboratory sampling, by moving an array of pipettes in a tray. A different carriage handled travel on each axis (X, Y and Z), and all of them had the same standard preload. Set-up was taking a lot of time and effort, the engineer reports. “We visited and looked at the application, and said, ‘You don’t need three standard products; the Z-axis travel is not that accurate. You can use a clearance carriage.’ And we bolted one in, put the carriage on and it worked.”
In this case, first appearances can be deceptive. “When a customer takes a standard carriage out of the box and move it on the rail, there’s minimal friction, and you think that’s fine. But when you put the two together, and interface with a structure that is not fantastically well-machined to achieve a certain price point, it performs differently,” he adds.
In this case, IKO offered the customer a product swap; it took away the unused carriages and swapped them for a product with greater clearance, all for the same price.
The Sales Engineer recalls that the customer’s reaction was: “‘Wow, you saved me so much time and effort, and I’d never even thought about that.’ In hindsight, it’s an obvious fix, but when you’re in the middle of a problem and dealing with the stresses of production and everything else, you’re not always thinking clearly.”
Fortunately, IKO UK is there to help customers, both before orders, and afterwards too.
