Concentric Locking Designs
A “one-size-fits-all” bearing selection does not apply universally — especially when it comes to lock reliability and the locking types of mounted bearings. The trend in unit material handling applications has shifted from traditional setscrew locking designs to newer concentric locking solutions. Concentric locking results in more near-perfect concentricity, quieter operation, increased lock reliability, higher speed of operation, and less damage to the shafting. The results: improved equipment performance and increased uptime.
What is a concentric lock design?
When it comes to mounted ball bearing units, there are two types of concentric locking designs: the concentric locking collar and the adapter lock design. The concentric locking collar is more popular, based on its ease of installation and related reliability. Adapter lock designs for mounted ball bearings are less popular and require extra care during installation. An overtightened adapter lock will reduce the internal clearance and preload the bearing, resulting in premature failure. Conversely, the concentric locking collar includes a “C” clamp collar that compresses the tangs on the inner ring extensions. Locking the bearing in place is as simple as sliding the bearing onto the shaft and into position and then tightening the cap screw on the collar to the proper torque. As the installer tightens the cap screw, the collar collapses on the tangs, deflecting them and tightly compressing the bearing inner ring to the shaft. Always please remember to follow the manufacturer’s recommended installation instructions.
For mounted roller bearings, concentric locking collars are not available because the bearing capacity and application loads greatly exceed the holding capability of a “C” clamp collar. Adapter lock designs are the only concentric option for mounted roller bearings; these consist of a simple sleeve, nut and washer (SNW) combination with a tapered bore inner ring bearing. Unlike a typical SNW, adapter mounted roller bearings often unitize the sleeve, nut and inner ring, so there are no loose components; the installer only needs to slide the bearing on the shaft and follow the installation steps. The installer drives the bearing up the sleeve in either a push or pull action by rotating the nut, which collapses the sleeve and locks the bearing to the shaft. Again, always please remember to follow the manufacturer’s recommended installation instructions.
What is near-perfect concentricity?
Mounted ball and roller bearings lock to shafts differently than unmounted bearings. An unmounted bearing is pressed onto a shaft to form a tight fit. Often, presses are used to slide a bearing onto the shaft; in other cases, bearings are heated to expand them before being slid onto the cooler shaft. Designers typically incorporate a shaft shoulder for the bearing to butt up against and use a lock nut on the other side to lock the bearing in place. Mounted bearings are designed to slide onto a shaft with no need for presses or heat; the bearing should slide along the shaft freely. Once the bearing is on the shaft, it needs to be locked into place. A common locking method uses setscrews that bore into the shaft to hold the bearing in place. (Note: mounted roller bearings utilize a locking collar that has setscrews, while mounted ball bearings have the setscrews in the bearing inner ring.) The setscrew can dig into the shaft and push the bearing off center. This means with each rotation, the bearing center can wobble from the centerline of the shaft, leading to a reduced holding force, increased noise, and possible bearing life reduction. Concentric locking designs dramatically reduce these effects, resulting in smooth running and quieter operations. Even with concentric locking designs, however, the impact of inner ring distortion caused by setscrew locking can reduce bearing life by increasing internal bearing stress.
How do concentric locks reduce noise?
This is a result of obtaining near-perfect concentricity. Less imbalance occurs due to closer concentric positioning during rotation. Imbalance creates noise, which can resonate within the steel structure. Often, the noise level is well below other unit material equipment components. But every potential decibel reduction makes it easier for a facility to meet Occupational Safety and Health Administration (OSHA) guidelines for ear protection.
How do concentric locks increase lock reliability?
Concentric locking designs provide 360-degree locking around the shaft, creating a friction grip that results in a much larger surface area for holding power compared to setscrew lock bearings. With setscrew locking, the holding force is controlled by two small setscrews that bite into the shaft and the opposite side of the inner ring bore that contacts the shaft. This results in less than 25–35% engagement between the inner ring and the shaft. Different manufacturers have setscrews at various spacings to help with holding force, as well as different heat-treating methods of the inner ring, which impact how well the setscrews hold. Additionally, different setscrew tip designs also affect holding force. Knurled point, cup point, ball point and diamond-faceted all bite into the shaft differently.
How can concentric locks have a higher speed capacity?
Typically, bearing speeds are impacted mostly by seal type and the feet per minute (fpm) rating of the seal converted into revolutions per minute (RPM), as well as operating speed temperature. However, the locking type also can play a part in speed capability. A concentric lock’s increased grip to the shaft, plus the near-perfect concentricity of the bearing to the shaft (reducing imbalance and overall heat generation internally), contribute to the bearing’s ability to operate at higher speeds in comparison to a setscrew lock.
How does concentric locking result in less damage?
While the use of setscrews may be managed correctly to avoid either all or some of these problems, it is still a less desirable approach. Eliminating the burrs in the shaft of the setscrews eliminates most of the shaft damage. As the setscrew bites into the shaft, shaft material deforms, causing a raised burr. When removing the bearing, the raised burr creates bearing bore and shaft scoring because the bearing must be forced off the shaft. Additionally, fretting corrosion shaft damage is reduced with concentric locking. Fretting corrosion is the result of the oscillation between two mechanically attached surfaces which then oxidates, giving the appearance of rust. This type of damage can be caused by a non-concentric ball path, due to the micromotion between the shaft and inner ring.
While a concentric locking design has its benefits and its user usage has grown, user preference continues to be setscrew locking. When it comes to installation, many users like the simplicity of setscrews: tighten two setscrews and you’re done. But it’s not that simple. Did the installer properly torque the setscrews to the bearing manufacturer’s recommended torque value? Is the setscrew properly stressed in the hole to provide the required holding force? Did the installer align the setscrew locations for both bearings on the shaft? If the setscrews are not aligned, more wobble is introduced in the system, resulting in less lock reliability, more noise and the possibility of locking failure.
When selecting a bearing for your unit material handling equipment, we believe a concentric lock design is the best answer for your solution.
This article first appeared in Modern Materials Handling on 4/6/2020