THKSHW21CAN1SSWide Low-Profile Caged-Ball LM Guide Carriage Block, Size 21 for 37 mm Rail, 21 mm Height, High MC Roll Rigidity, Block Only

Product Description

What this part is

This is a THK LM Guide carriage block, model SHW21CA — a wide, very low-profile caged-ball linear bearing block running on a 37 mm wide SHW size-21 rail. Four rows of balls at a 45° contact angle give four-way equal load capacity.

Why the SHW series exists — wide rail, low height, high MC:
The whole design premise is in the proportions. Assembly height is only 21 mm while the rail is 37 mm wide and the block 68 mm wide. In THK’s own words, “the Model SHW, which has a wide LM rail and a low center of gravity, is optimal for locations requiring compactness and a large MC moment rigidity.”

MC is the moment about the axis of travel — the rolling or twisting couple. Because the SHW’s raceways are spread far apart across a wide rail, a single SHW block resists that couple far better than a tall narrow block of similar capacity. Look at the numbers: MC is 0.229 kN·m against MA and MB of 0.0806 kN·m each, so roll resistance is nearly three times the pitch and yaw figures. That is unusual and it is the reason to specify SHW.

Where this pays: a single-rail axis carrying a wide, off-centre or cantilevered load, where you would otherwise need two parallel rails purely to stop the table rolling. One SHW rail can replace two narrow rails, which saves the second rail, its machining, and the parallelism alignment between them. It is also the series to reach for under a low overall height budget — 21 mm total is very shallow.

Caged ball, not full complement

Unlike THK’s HSR and SR series, the SHW uses a caged ball design: the balls are held in a synthetic cage so they never touch each other in the circulation path. The practical differences are real. Ball-to-ball contact is what generates most of the noise and heat in a full-complement guide, so a caged-ball block runs quieter and cooler, tolerates higher speed, holds grease in the cage pockets for longer relubrication intervals, and gives smoother, more consistent running friction. The trade is a slightly lower load rating for a given envelope, because the cage occupies space that balls could have.

Block only — and why that is fine here:
This is the block alone, with no LM rail. Normally that matters: THK serial-number-matches standard blocks to their rails and states that “an LM rail and LM blocks used in combination must have the same serial number.” This part carries the (GK) designation, and THK’s GK series exists specifically so blocks and rails can be bought separately — the GK line “features a lineup of interchangeable LM blocks and LM rails that can be combined freely as long as they have the same model number.” Fit it to any SHW21 GK rail of matching grade and the assembly is correct.

Decoding the designation

SHW — wide, low-profile caged-ball LM Guide series21 — size, corresponding to a 37 mm wide railCA — block type: flanged with tapped holes, mountable from the top or the bottomN — appears in the source part number SHW21CAN1SS; not a code THK publishes in the SHW model-number scheme, so it is retained as a marking rather than decoded1 — THK’s coding field for the number of blocks used on the same rail. Not a pack quantitySS — end seal plus side seal. Not “double seal”: DD is double seal, UU is end seals onlyGK — the interchangeable block-only / rail-only series

Published dimensions and ratings

Assembly height M: 21 mmBlock width W: 68 mmBlock length L, with UU or SS seals: 59 mm; block body L1: 43.6 mmBlock mounting hole pattern B x C: 60 x 29 mm; taps 6 x M5; through-hole ø4.4 mmMating rail width W1: 37 mm (0 / −0.05); rail height M1: 11 mmRail hole pitch F: 50 mm; hole d1 x d2 x h: 4.5 x 7.5 x 5.3 mmBasic dynamic load rating C: 8.24 kNBasic static load rating C0: 12.8 kNStatic permissible moment MA: 0.0806 kN·m (one block) / 0.434 kN·m (two blocks)Static permissible moment MB: 0.0806 kN·m (one block) / 0.434 kN·m (two blocks)Static permissible moment MC: 0.229 kN·m (one block)Rolling elements: four rows of balls, 45° contact angle, caged ballBlock mass: 0.24 kg; rail mass 2.9 kg/mStandard rail lengths: 130, 230, 380, 480, 580, 780 mm; maximum 3000 mmAccuracy grades: Normal, H, P, SP, UP

Two honest caveats

First, a grease nipple is not fitted as standard on the SHW series; THK drills it at the factory on request, and the size-21 fitting is PB1021B. Check whether the block you receive has one before you design a lubrication line to it.

Second, THK’s own sources disagree on preload availability for size 21: the model-number coding page lists Normal, C1 and C0, while the size-21 attribute page lists only Normal and C1. Rather than pick one, both readings are stated here. The grade of this particular block is not marked in the source data.

Interchange guidance

Cross-references: SHW21CA1SS, SHW21CAN1SS, SHW21CA1SS(GK), SHW-21-CA1-SS, SHW 21 CA, SHW21CA, SHW21CA1UU, SHW21CAN1SSGK. Related block type in the same size: SHW21CR. Adjacent sizes: SHW17CA, SHW27CA.

Central Surplus also stocks THK HSR25CA, HSR30CA, HSR30B and SR35W blocks.