Linear guide rails

THK's SRG35R assembly has a 34 mm rail and a 59.1 kN basic dynamic rating on a 100 km roller life basis. THK's factor of 1.23 expresses the same rating as about 72.7 kN on a 50 km basis. Its basic static rating is 119 kN. Compare guides on a common life basis and check the actual force and moment on each block. SRG, printed pp. 29–30.

Rail, block and load path

Loaded rolling elements carry reactions between the block and rail. Return passages circulate them. THK uses four rows of caged balls in SHS and caged rollers in SRG. THK SHS, Structure and Features, THK SRG catalog, printed p. 5.

Loads pass through block, rail, fasteners and mounting faces into the base. A separate drive supplies motion and has its own thrust and control requirements.

Balls and rollers differ in idealized contact geometry: point contact versus line contact. Thomson uses that distinction to explain its 500 Series roller guides. Under load, elastic deformation creates finite contact areas rather than mathematical points or lines. Product geometry, contact arrangement, preload and support conditions also matter, so this distinction is not a universal roller-to-ball capacity, speed or friction ratio. Thomson, 500 Series Roller Linear Guides.

This article concerns recirculating profile guides. Round-shaft bearings and plain sliding guides are other guidance architectures; the SRG dimensions and ratings below do not describe them.

Forces, moments and block arrangement

For SRG, THK specifies 45° contact angles and equal ratings in the radial, reverse-radial and horizontal directions. SHS also has four-way equal ratings, but that property belongs to the named contact arrangements. Equal directional ratings are not four independent capacities available simultaneously. SRG, p. 5; SHS features.

Force and moment have different units and effects. An overhung load creates moments about the guide's axes, while acceleration, deceleration and impacts can increase peak reactions. THK's static-selection guidance explicitly considers sudden starts/stops and overhang moments; a “static” check can therefore matter while the machine is moving. THK, Static Safety Factor.

Illustrative mechanics: a 1,000 N transverse force acting with a perpendicular 0.10 m offset produces M = Fe = 100 N·m = 0.10 kN·m about the corresponding axis. This is an assumed load and geometry, not a permissible guide load. Several blocks or two rails can redistribute reactions, but equal sharing does not follow from block count alone. Rail spacing, block spacing, table stiffness and mounting errors affect the distribution; combined forces and moments need a consistent load model.

Static rating, dynamic rating and fatigue life

The basic static rating C0 uses a reference permanent-deformation criterion. THK defines it for a constant-magnitude, constant-direction load giving combined permanent deformation of the rolling element and raceway equal to 0.0001 times the rolling-element diameter at the most highly stressed contact. It does not mean zero deformation or an acceptable operating deflection. The basic dynamic rating C instead enters rolling-fatigue life calculations. Neither number alone is an allowable machine payload. THK rating definitions.

THK defines nominal L10 life as the travel reached without fatigue flaking by 90% of identical units run individually under the same conditions. Its cited convention uses different reference distances and exponents for ball and roller guides:

Ball guide:   L10 = 50 × (C50 / P)^3       km
Roller guide: L10 = 100 × (C100 / P)^(10/3) km

P is the calculated load and has the same force units as C. The reference distance belongs to the rating definition; it is not a maintenance interval. THK warns that these formulas may not apply when stroke length is less than or equal to twice the block length. THK, Nominal Life; SRG, p. 19.

The SRG catalog prints rounded conversion factors C100 = C50/1.26 for balls and C100 = C50/1.23 for rollers. Changing the distance basis changes the stated rating number, not the product. Modified-life calculations additionally incorporate hardness, temperature, contact and load factors; conversion to hours also requires travel per cycle and cycle frequency. Nominal fatigue life is not a guarantee against contamination, lubrication failure, corrosion or every other service failure. SRG, pp. 19–20.

For a maximum-load check, the catalog gives the static safety-factor relationship fs = C0/Pmax. Its guideline lower limits are 2 without vibration/impact and 5 with vibration/impact. These are attributed supplier guidelines, not universal application acceptance limits or permission to use the reciprocal as an allowable payload. SRG, p. 19.

Bounded assembly: THK SRG35R and size-35 SRG rail

The example is the caged SRG35R block with its corresponding size-35 SRG rail in CATALOG No. 270-20E, printed code 202503. The contiguous SRG-R dimensional table identifies the compatible model family; this is not a complete configured ordering code. SRG, printed pp. 29–30.

These dimensions are catalog values, not measured parts; only the stated rail-width tolerance is assigned here. Model size “35” does not mean a 35 mm wide rail. The second stacked rating line belongs to the uncaged, full-complement SRG35GR and is excluded from this example. The table also gives MA and MB of 10.1 kN·m for two blocks in close contact; those values are not the one-block ratings and do not describe arbitrary spacing or combined loading. SRG table and footnotes, pp. 29–30.

Derived rating conversion: 59.1 × 1.23 = 72.693 kN, displayed as about 72.7 kN on the 50 km basis using THK's printed, rounded roller conversion. The extra arithmetic digits are not additional source precision. This does not improve the guide's physical capacity. For the 125 mm UU/SS block length, twice the length is 250 mm; the nominal-life formula caveat consequently includes strokes at or below 250 mm. A longer stroke alone does not establish all other model conditions. SRG, pp. 19 and 30.

A 1,000 mm size-35 rail is among the listed standard lengths. The catalog separately advertises SRG-GK interchangeable blocks and rails; that distinct family does not establish arbitrary interchangeability for this SRG35R assembly or cross-manufacturer compatibility. SRG, p. 17 and back cover.

Preload, accuracy and mounting geometry

Preload and accuracy grade are separate properties. For size 35, THK gives radial-clearance intervals of −2 to −1 µm for normal/no symbol, −3 to −2 µm for C1 light preload and −5 to −3 µm for C0 medium preload. Negative clearance is not positive free play. The C0 preload code is also distinct from the basic static rating C0. The cited heavy/super-heavy options are restricted to specified C/LC/SLC variants, rather than this R block. SRG, p. 15.

In the size-35 H accuracy grade, the assembled-height M tolerance is ±0.04 mm and width W2 tolerance ±0.03 mm. H running parallelism for a rail longer than 800 mm and no longer than 1,000 mm is 9 µm, evaluated as the block travels along a rail bolted to a reference surface. This is a grade- and length-conditioned guide property, not measured installed-axis positioning accuracy. SRG, pp. 15–16.

The size-35 horizontal mounting-error references between two rails are approximately 14 µm normal, 10 µm C1 and 7 µm C0 under normal use. Those qualifications distinguish reference mounting-error values from dimensional tolerances or measured alignment. Datum shoulders and corner relief also depend on model and accessories. The rail/block mounting faces, fasteners and supporting structure influence load concentration; no universal bolt torque, rail-parallelism tolerance or base-flatness recipe follows from these tables. SRG, pp. 16, 18 and 34.

Lubrication, contamination and thermal boundaries

SRG UU denotes end seals; SS adds side and inner seals. For size 35, the catalog specifies 30 N maximum SS seal resistance per lubricated block. It is not a constant drag measurement or total guide running resistance. Foreign material can cause abnormal wear and shorten life; sealing options do not establish immunity to every chip, coolant or environment. SRG, p. 7.

The catalog distinguishes anti-rust oil from operating lubricant and requires lubrication before use. It warns against mixing lubricants and notes that mounting orientation can obstruct lubricant distribution. Very short strokes can inhibit oil-film formation. Its replenishment recommendation is approximately every 100 km travelled (3–6 months), with the actual interval and amount determined by machine conditions and environment. This is not a fixed universal schedule or a maintenance-free guarantee. SRG, p. 34.

The same source cautions against external temperatures above 80°C, except for a heat-resistant type. That exception does not identify a heat-resistant configuration for this example or supply its full temperature range. It also warns that insufficient mounting rigidity or accuracy can concentrate load and reduce performance. Engineering synthesis: thermal growth of the base, rail and table can change geometry and internal loading; an accuracy-class label cannot compensate for the machine's thermal and structural behaviour. SRG, p. 34.

Source scope and unresolved application conditions

The THK reference is Caged Roller LM Guide SRG, CATALOG No. 270-20E / 202503, identifying March 2025 without an issue day. PDF creation and modification metadata are separately dated March 10 and March 13, 2025, in +09:00. The URL filename, metadata and HTTP dates do not establish a technical edition. The cited HTML guidance has no identified substantive edition dates.

Coverage is limited to the manufacturer's catalog and technical guidance. It does not include the full ISO 14728-1 or ISO 12090-1 standards or an independent assessment of standards compliance.

Application-specific block arrangement, preload and accuracy selection, load spectrum, duty, mounting structure, lubricant, contamination exposure and temperature history remain outside this example. The example establishes catalog identity and source-specific relationships; it does not establish installed accuracy, life, interchangeability approval or a machine operating envelope.