Lubrication, loads, direction, speed, and distance determine which guide component — specified in terms of size and number of bearing assemblies as well as ring size — is best for a given system load capacity and life. For longer life, systems should be designed for loads higher than those to be carried during normal operation.
For HepcoMotion's PRT Precision Ring and Track systems from BishopWisecarver, we calculate system life in three steps:

Resolve loading on system into direct and moment load components

Obtain the system load factor L_{F}

Apply the load factor to the appropriate nomagram to determine system life.
Load components affecting a carriage traveling on curved track are different than for rings rotating around fixed bearings — so different load factor equations are required to determine system life.
Carriage capacity and life on rings, curved track
When calculating life for a curved track system, loading on the system must be resolved into direct load components L_{1} (axial loads parallel to bearing shaft) and L_{2} (radial loads perpendicular to bearing shaft), and three moment load components: M_{S} (roll), M (pitch), and M_{V} (yaw). Centrifugal force affects L_{2} and M_{S}, because it moves in a radial direction, a force spiraling away from the movingobject center of mass (COM).
COM force is calculated F = DV^{2}/R, where V is COM velocity (in m/sec), R its distance from the ring axis (in m), and D its mass. F is in Newtons. Next, we obtain main load factor L_{F} with respect to duty cycle:
where maximum load capacities are obtained from the system manufacturer. Then, the direct and moment loads of the track components and type of carriage must also be identified.
Capacity and life for rotating ring systems
In applications where a ring rotates around fixed bearings, assemblies should be equally spaced around the ring. (Where bearing assemblies rotate with load, assemblies can be spaced unequally.) Loading must be resolved into the two directload components (axial loads parallel to the ring axis L_{A} and radial loads perpendicular to it L_{R}) and the roll moment load component M.
As with carriages on curved track, centrifugal force affects factors L_{R} and M. Here, the main load factor L_{F} is:
Assume we have one 360° ring with a 25mm crosssection (and 351mm diameter) that rotates along six RLJ25 fixed bearing assemblies. Also assume that the ring rotates once per second, has five lubricators, and that:
• Rotating assembly (ring, platform, payload) is 8 kg • COM is 100 mm from the ring axis, and 150 mm above the ring Vs • Duty cycle is 36 hours per week. Axial, radial, and moment loads are then resolved:
Axial load: L_{A} = 8 kg × 9.81 m/sec^{2}(g) = 78.5 N
Center of mass speed: 1 rev/sec = 2 × š × 0.10 m × 1 = 0.63 m/sec
Radial load: L_{R} = DV^{2}/R = 8 kg ×(0.63 m/sec)^{2} ÷ 0.10 m = 31.8 N
Moment load: M = L_{R} × h = 31.8 N × 0.15 m = 4.77 Nm
From Table 1: M_{max} = (187 + 2 × 37) × Øc = 261 × (0.351 + 0.020) = 96.8 Nm
L_{Amax} = 750 + 2 × 150 = 1,050 N
L_{Rmax} = 400 + 2 × 100 = 600 N.
Load capacity tables give corresponding direct and moment loads.
To obtain linear life, a 0.177 life factor value (calculated with the above equation) is entered into a load/life nomogram (shown above) for lubricated systems. The factor 0.177 corresponds to 39,000 km on the nomagram, which predicts system life in kilometers traveled.
For more information, visit bwc.com.
About carriage types
There are two common carriage types for ring or curvedtrack guide wheel systems. Properly specified, they can move bidirectionally. Fixedcenter carriages are typical for track systems of common bend radii without S bends, ring slide tracks, and segment tracks. Bogie carriages accommodate S bends and varying radii and (because of wide bearing spacing) they improve stability. A bogie carriage swivels on a selflubricating axial/radial bearing with adjustable preload, for traversing straight and curved track joints without the clearance seen in fixedcenter carriages.
Fixedcenter carriage geometry enables traversal from straight to curve track section, allowing each pair of bearings to follow slides independently. Where bearing assemblies traverse straight and curved track joints, there's a small amount of play — but it usually doesn't affect system operation.