Unique lead screw leads to quick linear motion system
November 1, 2005 by Peter Ratcliffe
In today’s high-paced society, speed is a way of life. For case packing machines, whereby a machine or person moves a product down a conveyor to be packaged, it is important for manufacturers to move a large volume of products in a short amount of time. The case packer must not only be fast, but also cost-efficient and durable enough to ensure a long lifespan.
For the past 24 years, I’ve been the president of RPT Motion, a Quebec-based provider of custom modular linear motion systems. During the past few years, we have experienced an increasing demand for case packers and other linear motion solutions from many different manufacturing customers.
So much of our business is driven by speed. Our customers have always looked to us for ways to speed automation, but in light of the recent globalization, we have observed an intense pressure to provide systems that reduce downtime and increase overall productivity.
A key segment of RPT’s business is based on updating existing subsystems on machinery with higher performance linear motion to increase production rates and reliability.
Although speed is a critical component for linear motion applications, size plays a significant role as well – that is, increasing speed without affecting a machine’s or sub-assembly’s dimensions. This is because changes in dimension necessitate redesign, and redesign costs money.
Recently, RPT Motion teamed up with Kerk Motion Products, a manufacturer of non-ball lead screws, for a challenging retrofit project involving a major tissue manufacturer whose operation was getting bottlenecked at the case packing stage. The manufacturer was looking to add a second case packer to its process to increase the speed and prevent the slowdown of its current production.
The process required more than just dropping in the new case packer. The first obstacle was the existing system, which had a two-position lane changer powered by a pneumatic cylinder. It wouldn’t provide the four required positions or the higher speeds. To service the four lanes, a stroke in excess of 40 inches was required and the lane changer needed to index on average every 10 seconds or less, while maximizing time available for product flow.
At first, we couldn’t fit the ball screw and nut into our existing design with a fast enough pitch to suit the higher speeds. It would hit its limits and not allow us to reach the high linear speeds we needed. We could have found a ball screw with a fast lead, but it would not have fit into the current assembly, forcing us to redesign the entire slide system for a bigger, heavier screw.
To tackle this motion control challenge, both companies helped design a motion system to feed the required four lanes (two lanes in each of two case packers). Our two companies collaborated to include a Kerk screw and nut in the RPT Modular Slide System, a smooth running, high-speed, servo-powered mechanism, with an Anti-Backlash Lead Screw Assembly.
The new system featured a VHD Series of lead screws, which come in a variety of different leads in the same size physical package. Components to mount the nut, as well as to support and drive the screw, remained the same over a huge choice of feeds and speeds, and therefore potential uses. In order to meet the high speeds required in this lane changer application, RPT chose a 2.4 inch travel per revolution screw of 3/4-inch nominal diameter, which proved to have approximately the same efficiency as a ball-nut configuration.
The physical size of the VHD high lead screw and nut package was also much smaller than available ball screws of similar leads and this contributed significantly to the overall success of the project.
Kerk is always looking for ways to meet the needs of its customers. According to Dave Arguin, sales manager at Kerk, “At the time we worked with RPT Motion, the 2.4-inch lead was the longest we produced. We currently offer a much higher 92-mm (3.625-inch) lead on the same diameter screw.”
The servo-powered system achieved the speeds necessary to accommodate the second case sorter and subsequently eliminate the manufacturer’s bottleneck–within the confines of the system’s original dimensions. The lane changer initially functioned without failure or even noticeable backlash for three years.
The use of the self-lubricating VHD Series was also instrumental in combating environmental challenges. Since airborne tissue paper dust is regularly generated in the cartoning and case packing area, almost everything eventually gets covered in a light layer of dust. Grease, which captures dust and creates a harmful, abrasive paste, is eliminated in this process, because the VHD screw does not require grease to operate. Additionally, there are no greasy ball circuits to be damaged by the dusty paste formed on the screw.
Also, the anti-backlash properties of the screws have proven to be a particularly valuable feature in the system. Any wear that does eventually occur in the screw or nut is compensated by the patented Kerk mechanism, which adjusts automatically to maintain a zero backlash over a very long service life.
Noise reduction was another benefit. Because Kerk’s lead screws do not rely on balls or ball returns to generate noise, the new system runs silently, making for a much more pleasant and safer working environment.
The Kerk VHD system has become the first choice of RPT Motion when addressing linear motion systems. It has proven significantly lower cost in most systems than either ball screw or timing belt drives. This VHD is more predictable and easier to tune than a timing belt powered system and has similar performance at lower initial cost than a ball screw.
Because of the lead screw design solution, RPT Motion was able to deliver a fast, high-quality case packer machine at a competitive price.
Peter Ratcliffe is the president of RPT Motion Inc.