Linear Guides in Motion Design

Definitions and common linear-motion variations

This first section breaks down the main types of linear guides and their subtypes along with a comprehensive look at the standard industry terminology used to describe them.

Sliding element linear guides consist of a carriage or slide that rides over a surface known as a rail, way, or guide. Sliding contact occurs when the moving part directly contacts the rail section.

Rolling element linear guides are either recirculating or non-recirculating.

• Non-recirculating types apply rolling elements such as bearing balls or rollers. They pair profiled wheels with precisely mated tracks — usually V-shaped, rounded, or C or U-channel. These types can be called guide wheels, roller guides, cam rollers, and track-roller guides. Benefits include high load capacity, good stiffness, and smooth motion.
• Recirculating rolling elements like profile rail employ a bearing block or carriage containing small internal raceways in which rolling elements move the block along a profiled rail. Benefits include high strokes and low friction in cleaner environments, but can be costly for consumer-grade products.

Simplicity linear slide assemblies use sliding contact between self-lubricating Frelon^® bearings and round rails.

Ambiguous linear-guide terms

This chapter differentiates several technical definitions as they relate to track-roller linear guides and profile rail linear guides.

Gothic-Arch Linear Bearing (4 points of contact): Gothic arches have greater clearance and roller-to-rail contact that provides exceptional linear accuracy, but can make for lower load capacities.

• Gothic arches in track rollers refers to the roller wheel’s outer working-surface shape and the mating geometry of the linear track.
• Gothic arches in profile rail linear guides are found in the geometry of the linear rail raceways and not the carriage or rolling elements.

Roller Bearings: The term roller bearing generally refers to rotary bearings with cylindrical roller load-bearing elements instead of spherical ball load-bearing elements.

• Roller bearings in track rollers can refer to the entire linear-guide design or just the rolling wheel or cam follower of the design.
• Roller bearings in profiled rail refer to permutations of cylindrical or barrel-shaped rollers as opposed to spherical balls for the carriage’s loadbearing elements.

Rail: The term rail is often a casual way of referencing track-roller linear guides, or it can imply profiled rail with its pairing of carriage blocks. In other contexts, rail might specifically imply the track geometry and its surfaces that serve as the wheel raceways.

Low Profile Redi-Rail is a low-profile track-roller option that employs Gothic arch wheels running on hardened steel races. Can handle loads beyond 100 lb.

Core design inspiration: Cam followers

Cam followers are power transmission devices with a rotary bearing core that bears the load (both radial and axial) while serving as the interface between independently moving machine sections. Physical separation between these sections is maintained to minimize rotational friction.

Common cam-follower designs employ thin cylindrical-shaped needle rollers for higher radial loads and speeds. Cam followers with twin rows of standard ball-bearing elements are good for high loads where the axis needs high dynamic load capacity. Both are ubiquitous in automated storage and retrieval systems.

Track-roller linear guides: Where they excel

Track-roller linear guides often serve in two challenging application types including long-stroke linear-motion axes and linear systems in harsh environments. Below is a quick list of five track-roller benefits:

• High speeds due to radial bearing elements.
• Light weight aluminum carriages and rails.
• Effective in harsh environments as opposed to profile rail.
• Easy preloading adjustments and simplified replacement.
• Less stringent mounting demands for ease of installation.

Cutaways of Redi-Rail wheels and shown assembled into the mating track (left)  as well as Hevi-Rail wheels (right).

Track-roller wheel design on performance

Track-roller wheels can be customized to an application through their internal bearing-element arrangements, sealing, and outer tread geometry and material makeup. Variations in wheel geometries and their benefits include:

• Flat wheels: Heavy-duty automation and material handling applications.
• Crowned (rounded) wheels: Slightly rounded radial profile is more costly but allows for modest misalignment while helping to avoid corner loading.
• V-shaped (notched) wheels: Can operate at very high-accuracy axes and resolve radial and thrust loads while shedding debris in dirty environments.
• Chamfered wheels: V-shaped and tapered profiles avoid corner loading in heavy-duty applications such as fork trucks and other lifts.
• Flanged wheels: One or both axial sides have a wide flare which serves to center the wheels on the raceway.
• U-groove (Gothic Arch) wheels: Often paired with hardened and ground steel raceways, they are highly engineered for secure and smooth traversal of reciprocating axes and vertical lifts.

Profiles of different geometries for track-roller wheels.

Track and carriage installation on performance

The tracks and carriages of linear guides provide innumerable options for tailoring the appropriate performance to the application.

• Hardness: Hardened tracks typically support softer wheels, in contrast to aluminum wheels that can pair with engineered polymer rollers carrying lighter loads
• Surface straightness, flatness, and parallelism determine system accuracy, yet linear guide tracks can bolt to moderately inconsistent surfaces without exhibiting running issues.
• Preload can eliminate play between load-bearing wheels and tracks.
• High-precision track mounting requires stringent mounting-surface preparation and techniques to achieve published accuracy, straightness, and rigidity values.
• Bogie carriages: Their main purpose is to bear load while allowing wheels sets to stably ride on both straight and curved sections of track.
• V-shaped tracks use flat wheels to engage the two sides of a V-shaped track, excelling in dirty environments with abrasive debris and those that require frequent reversals.
• Roller pillow blocks consist of a stationary pedestal block and a free-rotating assembly to provide friction-free support of a shaft.

Dynamic and static load capacities for track-rollers

Load-capacities, which can be computed into static and dynamic values, are central to the specification of linear guides. Real-world influences on wear and life can include:

• Accelerated wear or fatigue from operating clearances.
• Extreme temperatures, vibrations, shock loads, or contamination.
• Short-stroke reciprocating motions that cause false brinelling.
• Unanticipated damage during installation and mounting.

Roller pillow blocks can deliver dynamic load ratings from 3,560 to 55,600 N and a low coefficient of friction allows speeds up to 7.6 m/sec.

Five things to consider in vertical applications

One of the core considerations when sizing and selecting a linear actuator is mounting orientation. Below are five factors that engineers should consider when designing for vertical duty:

• Loading: Gravity affects the bearing life and guides.
• Lubrication: Gravity affects consistency across the entire vertical axes.
• Buckling load: Ball and lead screws can bend and buckle under vertical loads.
• Back driving: Screws can back drive depending on friction, lead angle, and efficiency.
• Contamination: Front and side seals should be used in vertical applications.

Repeatability, accuracy, and inertial values

There are numerous ways to classify the performance of linear-positioning devices such as ballscrews, belts, and rack and pinion systems. This section helps define the difference between accuracy and repeatability.

• Repeatability is the ability to return to the same position multiple times under identical conditions.
• Accuracy is the degree to which a measurement, calculation, or specification conforms to the correct standard.