High-precision linear ball bushing

01

Calculate rated life from load rating and factors

THK's selection guide defines the linear bushing's nominal life as L10 = (C/Pc)³ × 50 km, where C is the basic dynamic load rating and Pc the calculated load, and notes that the formula may not apply when the stroke is no more than twice the nut length. Ratings quoted for a 100 km life convert as C100 = C50 ÷ 1.26 under ISO 14728-1. The modified life is L10m = (α × C/Pc)³ × 50 km with α = fH × fT × fC ÷ fW. Service life in hours is Lh = L10 × 10³ ÷ (2 × stroke in m × reciprocations per minute × 60). [1]

THK's load factor fW is 1 to 1.2 for faint vibration at speeds up to 0.25 m/s, 1.2 to 1.5 for weak, 1.5 to 2 for medium, and 2 to 3.5 for strong vibration or impact above 2 m/s. [1]

Editorial calculation: with an illustrative C = 1,000 N and Pc = 200 N, L10 = 5³ × 50 = 6,250 km; at a 0.3 m stroke and 20 reciprocations per minute that is about 8,700 hours. With fW = 1.5 the life falls to about 1,850 km, or 2,570 hours. A rating of 1,000 N quoted on a 50 km basis equals about 794 N on a 100 km basis. [1] Ask each supplier which life basis its C value uses.

NB's slide bush catalogue uses the same 50 km form, L = (fH · fT · fC / fW · C / P)³ · 50, with life in hours Lh = L · 10³ / (2 · ℓs · n1 · 60). [2] Bosch Rexroth's linear bushing tables instead state that dynamic load ratings are based on a total travel of 100,000 m and that the C values are multiplied by 1.26 when based on 50,000 m. [3] [4]

Editorial calculation: Rexroth's standard 20 mm bushing has C = 1,170 N (minimum value) on the 100,000 m basis. At P = 200 N, (1,170/200)³ × 100 km ≈ 20,000 km; converting first, 1,170 × 1.26 ≈ 1,474 N on the 50 km basis, and (1,474/200)³ × 50 km also gives about 20,000 km. The life is the same; only the quoted C differs, which is why a 50 km rating looks 26% higher than a 100 km rating for the same bushing. [3] [1]

Give the supplier the load and its direction, moments, speed, stroke and cycle rate, required life, shaft diameter, tolerance and hardness, number of bushings per shaft and their spacing, open or closed type, seals, lubrication, temperature and housing.

02

Check shaft hardness, ball-row orientation and contact

THK says the raceway must be 58–64 HRC for full load capacity; below that both dynamic and static ratings fall by the hardness factor fH, which its chart shows dropping to about 0.5 at 50 HRC. Because a linear bushing's balls run directly on the shaft, ask for the shaft's hardness as well as the bushing's. Above 100 °C a temperature factor applies and above 80 °C a type with metal retainer plates should be used. [1]

Ratings in tables assume one ball row directly under the load; with two rows sharing the load, THK's multipliers range from 1.25 × C to 1.46 × C depending on the number of rows. When two or more bushings are mounted in close contact, multiply C and C0 by the contact factor: 0.81 for 2, 0.72 for 3, 0.66 for 4 and 0.61 for 5. [1]

Editorial calculation: since life varies with the cube of the rating, fH = 0.5 reduces life to 0.5³ = 12.5% of the full-hardness value. [1]

NB's notes on shaft selection recommend a hardness of 58 HRC or more and a surface roughness below Ra 0.4, and warn that the parallelism of two or more shafts strongly affects motion and life. Its normal choice is a clearance fit for the shaft, with a transition fit for higher accuracy; for its RBW type it names a g6 shaft for the clearance fit and h6 for the transition fit, and H7 for the housing when a flange bush uses its outer cylinder as the pilot. [2]

Rexroth shows how the fit sets radial clearance: for its Super a bushing on a 20 mm h6 shaft the clearance is +11 to +43 µm in an H7 bore, −7 to +25 µm in K7 and −14 to +18 µm in M7. For its standard bushing it recommends an H6 or H7 housing bore and determines radial clearance statistically from bore and shaft tolerances. [4] [3]

Open types: NB states that when the load acts constantly toward the opening, fewer ball rows carry it, and its Table C-8 reduces the basic static load rating to between 0.35C and 0.64C depending on size (3-row steel-retainer types excluded); Rexroth gives open-type ratings for the main load direction only. [2] [4]

03

Compare standard and high-capacity designs in the same envelope

Bosch Rexroth's 2015 linear bushing catalogue (R999000488) lists, for a 20 mm shaft, a standard closed bushing (R0600) and a Super a closed bushing (R0670) with the same outer diameter of 32 mm and length of 45 mm. [3] [4]

The standard bushing has a hardened and machined outer sleeve, a steel ball retainer (POM or PA for shafts of 3 to 10 mm) and 5 ball rows at 20 mm; it lists C = 1,170–1,390 N and C0 = 860–1,250 N, and weighs 0.100 kg. [3]

The Super a bushing has a PA or POM outer sleeve and retainer carrying hardened steel bearing plates with machined ball grooves, compensates misalignment up to 30′ and has 6 ball rows at 20 mm; it lists C = 2,020–2,470 N and C0 = 1,050–1,340 N, and weighs 0.061 kg. At 50 mm the two designs list C = 8,260–8,740 N and 12,060–14,730 N respectively. [3] [4]

Editorial calculation: at the same load the Super a's minimum C of 2,020 N is 1.73 times the standard's 1,170 N, which by the cube law gives about 5.1 times the rated life, at 61% of the weight. Check the rating basis and the load direction before comparing across makers. [3] [4]

NB groups its slide bushes by retainer arrangement: with the single type as 1, its comparison table lists dynamic ratings up to 1.6, static ratings up to 2 and allowable static moment up to about 21 for multi-retainer types, and it recommends double or triple types where a moment load applies. Its GM series uses resin sub-parts for a weight reduction of 30–50% against the SM series. [2]

04

Match materials, temperature, seals and lubrication

NB makes the standard slide bush outer cylinder from bearing steel and the anti-corrosion type from martensitic stainless steel, with steel (stainless for anti-corrosion) or resin retainers. Its operating temperature table gives −20 to 110 °C for steel with a steel retainer, −20 to 80 °C with a resin retainer, and −20 to 140 °C for stainless steel with a steel retainer, falling to 120 °C with a seal. [2]

This matches THK's advice to use metal retainer plates above 80 °C. [1] [2]

Seals: NB uses nitrile rubber seals, offers fluororubber seals for SM sizes 6 to 30 for high temperature, and a double-lip seal (outer lip against particles, inner lip against grease leakage) for SM(S) 6 to 30 with somewhat higher seal resistance. Rexroth's standard bushings without wiper seals allow higher temperatures from 12 mm shaft diameter. [2] [3]

Lubrication: NB ships slide bushes with anti-rust oil, asks for cleaning before lubrication, and recommends lithium soap-based grease with periodic relubrication. Rexroth's Super a bushings come without initial lubrication. [2] [4]

05

Linear bearings sold beside guides, screws and shafts

Limon, Asia Fon, ALM, VXO and LinRail list linear ball bearings or bushings alongside linear guides, ball screws, shafts and linear modules.

Compare their listings on rated C with its life basis, ball rows, retainer material and fit, as set out above. [3] [2]

06

Sleeve, ball rows, retainer and the shaft as raceway

The shaft sits in this list because the balls run directly on it: its hardness, not only the bushing's, decides whether the catalogue rating holds. [1]

07

Before quoting: rating basis, housing fit and radial clearance

Read the dynamic load rating row together with its life basis: the same bushing shows a C value 26% higher on the 50 km basis than on the 100 km basis, so two quotes can differ on paper without differing in life. [1] [3]

08

Send the load, motion and shaft data together

Paste it into your enquiry below or into an email to a maker, then replace every bracket.

Load: [force; direction; moments]
Motion: [speed; stroke; reciprocations per minute; required life]
Shaft: [diameter; tolerance; hardness; roughness]
Housing: [bore tolerance; required radial clearance]
Arrangement: [bushings per shaft; spacing; open or closed]
Environment: [temperature; contamination; lubrication]
Rating basis: [C at 50 km or 100 km]

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