Linear Bearings

Non-recirculating Linear Bearings

Non-recirculating linear bearings have balls or rollers that directly support loads and are contained in a housing. The rolling elements inside of the bearing rotate about there own axis and do not move within the housing. Since the bearings don't are fixed and don't move within the housing they can only move a fixed distance. Also they have a high precision and travel accuracy because the bearings only rotate in place and don't circulate through the bearing. Due to their high precision and accuracy the bearings are used in machining tools and measurement devices.

Rolling Elements

Ball Bearing Slides

• These bearings have similar construction to a rotational ball bearings. There are two raceways that the ball bearings sit in and a cage or retainer that spaces out the ball bearings apart and keeps them from colliding.
• Ball bearing slides are a type of non-recirculating linear bearings so the fixed housing around the ball bearings only allow of a finite distance travelled making them good for drawer slides.

Roller Bearing Slides

• Roller bearing slides can be made in two different ways creating a similar effect. The first method is the most familiar way with rollers that roll in two raceways contained by a retainer. The next way has two rails. One rail is c-shaped and creates two raceways. The other rail has wheel like rollers that are attached to the side of the roller and fit in between raceways of the other rail. 
• Since the wheels are fixed to the rail there is no need for a cage to space out the rollers. 
• Both of theses styles of roller bearings are non-recirculating and have a fixed distance of travel.

V-Type Roller Bearings

• These bearings have a a v-shaped groove cut into one one rail and a v-shaped indent cut into the opposite track. In between the tracks are rollers that held in a shaped cage.
• The rolling elements are fixed giving the bearings a finite distance to move. 

Crossed-Roller Bearings

• Crossed-roller bearings have roller that are set at alternate angles and offset from one another. The raceways are similar to the v-type roller bearings but instead of a one v-shaped groove and an opposite v-shaped mating piece the crossed-roller bearings have two v-shaped raceways for the crossed rollers to roll in.
• These bearings are also non-recirculating so the have a fixed distance of travel.

Recirculating Linear Bearings

This type of bearing can travel any distance because of how the ball bearings or roller bearings move throughout the bearing. In these kinds of bearings the rolling elements follow a circuit, a ball bearing will start at one point in the circuit and travel on the raceway up and around through the bearing and back onto the raceway. Since these bearing's rolling element recirculate this allows the bearing to roll in theory forever despite the small size of the bearing it's self. A problem with these bearings is there low accuracy cause by a pulsing force from the rolling elements. This pulsing occurs because the rolling elements go from a loaded state to a non-loaded state. A rolling element is considered loaded when it is in contact with the raceways and holding an applied load. it is considered to be non-loaded when it is not in contact with the raceways. Another draw back to using recirculating bearings is the maximum speed that can be achieved. As the rolling elements circulate they need to make a turn around the track, these turns are what cause forces that limit the max speed of the bearing. To increase the speeds ceramic rolling elements can used reducing mass of the rolling element and also reducing the force applied going around the turn. This however reduces the overall load capacity.

Rolling Elements

Profiled Rail Linear Bearings with Recirculating Balls or Rollers

• The balls follow a circuit. At any given point there are rolling elements in the load-carrying zone and a non load-carrying zone (return zone). The load-carrying zone is the portion with the bearing make contact with the profiled rail and the return zone is where the rolling elements are being recirculated to the load-carrying zone.
• The profiled rails allow for the rolling elements to easily rotate one it.
• These bearings can travel an infinite distance due to there recirculating properties.

Ball Screw

• Ball screws take rotational motion and translates it into linear motion. This works by having a ball screw and a ball nut attached to each other. The ball screw has a tracks that the ball bearings can sit in and circulate through. As the ball screw is turned the ball bearings move through the circuit and move the ball nut up or down the shaft of the ball screw.
• This bearing doesn't have a finite distance it can travel however it is limited to the length of the ball screw.

References

Contributors:

• Lesley Lang