A 0.250 in square key cut for a 1.000 in nominal shaft must stay within a width/height band of 0.249 to 0.252 in, with a 0.010 in corner radius, per BS 46 Part 1:1958 [S3]. The matching 1/4 in square keyseat in the shaft runs 0.250 to 0.251 in wide, the hub keyseat 0.250 to 0.251 in, and the shaft keyseat depth runs 0.142 to 0.148 in, half the key height plus standard clearance [S1][S3].
Length is the dimension the buyer or machine shop most often gets wrong on a 0.250 x 0.250 square key. IS 2048:1983 splits length tolerance into three bands: keys up to 28 mm long carry -0.2 mm, keys 32 to 80 mm long carry -0.3 mm, and keys 90 mm and longer carry -0.3 mm; keyways (the slot) carry the matching +0.2 mm or +0.3 mm on the high side to guarantee an assembled fit [S6]. A 1.37 in blank is 34.8 mm, so it lands inside the 32 to 80 mm band, meaning the key itself is allowed 0.3 mm undersize and the keyway 0.3 mm oversize relative to nominal. For a deeper walk-through of related shaft-hub connection choices, see the engineering selection guide on locking assemblies for shaft-hub fits in pulp and paper service, which sits one level up the assembly stack from a single square key.
Where the 0.250 x 0.250 Square Key Lives in BS 46
BS 46 Part 1:1958 maps square key geometry to shaft diameter ranges, and a 0.250 in square key is the standard size for shaft diameters from 3/4 in up to and including 1 in [S3]. The full keyway row in the imperial table reads: shaft keyway width 0.250 to 0.251 in, shaft keyway depth 0.142 to 0.148 in, hub keyway width 0.250 to 0.251 in, hub keyway depth 0.115 to 0.121 in, with a 0.010 in corner radius [S1]. The key itself is tabulated at 0.249 to 0.252 in on width and height, with 0.010 in corner radius [S3].
The preferred length range for a 1/4 in square stock under BS 46 is 3/4 in up to 3 in, so a 1.37 in blank sits inside the preferred window, not in the non-standard tail [S3]. Standard keyway sizes for ANSI B17.1 list the same 0.2500 in width and 0.2500 in height for the 0.8750 to 1.2500 in shaft diameter band, with a shaft keyseat depth of 0.1250 in and a hub keyseat depth of 0.1300 in [S5]. For a hands-on look at the broader role of keys and keyseats in mechanical power transmission, the industrial shaft-key reference covers the same fit-class logic in plain language.
Length Tolerance: What the Standards Actually Allow on a 1.37 in Blank
IS 2048:1983 is the cleanest source on length tolerance for a 1.37 in (34.8 mm) square key. For lengths from 32 to 80 mm, the key is allowed 0.3 mm undersize, and the keyway is allowed 0.3 mm oversize relative to nominal [S6]. Translated to inches, that is roughly 0.012 in of negative tolerance on the key and 0.012 in of positive tolerance on the slot, so a 1.37 in key can ship as short as about 1.358 in while still meeting the standard, and the matching keyway can be cut as deep as 1.382 in.
ANSI B17.1 does not pin a single length tolerance number the same way; it treats key length as a derived dimension driven by shaft length and hub length. The default rule of thumb, also used by the RivCut reference, is to size the key at roughly 1.5 x shaft diameter, so a 1.000 in shaft points to about 1.50 in of key length [S5]. A 1.37 in blank falls a little short of that 1.5 in target, which is acceptable provided the key engages both the shaft keyseat and the hub keyseat along its full length with no axial float. Keyseat alignment is independently limited to 0.010 in maximum offset and 0.002 in maximum lead at right angles to the shaft centerline, per ANSI B17.1 [S2].
Fit Class vs. Length Tolerance: Two Different Knobs
Fit class controls side-to-side clearance, not length. ANSI B17.1 names three fit classes: Class 1 is a clearance or metal-to-metal side fit using broad negative-tolerance bar stock, Class 2 is a normal side fit with possible interference or clearance using plus-toleranced key stock, and Class 3 is a deliberate interference fit that is not tabulated but typically rides the top and bottom limits of Class 2 [S2]. Width tolerance for a Class 2 normal fit is roughly +0.002 in / -0.000 in on the keyway, which is the band to design against if the spec just says "square key, normal fit" [S5].
Length tolerance runs on a separate track from side fit. The key can be held to the full +0.000 / -0.012 in length window under IS 2048 and still be a sloppy Class 1 side fit, or it can be a tight Class 2 side fit with the same length window. The two stack: a buyer who wants a 1.37 in key with a Class 2 side fit should write the length to nominal (1.37 in) with a -0.012 in tolerance per IS 2048, plus a +0.002 / -0.000 in width band per ANSI B17.1 Class 2. A point often missed: coupling hubs and similar components frequently use metric keyways to ISO/R773 with a Js9 width tolerance, which the supplier can substitute if the spec is left open, and the surface finish on keyway sides and bottom is held to 250 microinches max [S4].
What Goes Wrong on a 0.250 in Square Key at 1.37 in Length
Three failure modes show up repeatedly when a 0.250 in square key is ordered to 1.37 in length. First, undersize length: a 1.37 in key cut to 1.350 in (the practical floor from a stock bar) still meets the IS 2048 -0.3 mm tolerance band on its own, but it loses engagement length in the hub keyseat and concentrates shear stress at the shaft keyseat exit. Second, oversize hub keyway: a keyway cut to 1.40 in long will swallow the key, leave an axial gap, and let the hub walk under reversing load. Third, mixing standards silently: a BS 46 square key (width 0.249 to 0.252 in) dropped into an ISO/R773 Js9 metric keyway (roughly 6.000 to 6.015 mm on a 6 mm key) will not seat on width even though both are nominally 0.250 in class. [S5]
Material selection also bites if the spec stops at "square key." Plain mild steel key stock is rated for the safe working load given by the BS 46 relationship: torque in inch-pounds equals 9,000 x length x F x shaft diameter in inches, with horsepower as torque x RPM divided by 63,025 [S1]. For a 1.37 in key on a 1.000 in shaft at an allowable shear stress F around 30,000 psi (typical for medium-carbon key stock), the safe torque transmission is on the order of 9,000 x 1.37 x 30,000 x 1.0 = roughly 3.7 x 10^8 in-lb theoretical, derated sharply by the actual fillet, surface finish, and whether the load is steady or reversing. A key that looks identical on the print can carry a 2x to 3x torque spread just from the material grade, which is why a growing number of buyers in driveline and coupling service now write the material and hardness on the same line as the size.
Reading the Spec: BS 46 vs. ANSI B17.1 vs. IS 2048 Side by Side
BS 46 Part 1:1958, ANSI B17.1, and IS 2048:1983 are not interchangeable on every line, and a single 0.250 x 0.250 x 1.37 in call-out can be answered correctly three different ways. On width and height, BS 46 lists 0.249 to 0.252 in for the key, ANSI B17.1 lists 0.2500 in nominal with a Class 2 keyway width tolerance near +0.002 / -0.000 in, and IS 2048 carries the metric equivalent for a 6 mm square key [S1][S3][S5][S6]. On keyseat depth, BS 46 reads 0.142 to 0.148 in on the shaft and 0.115 to 0.121 in in the hub, ANSI B17.1 reads 0.1250 in shaft and 0.1300 in hub as the nominal targets, and IS 2048 mirrors the metric depth band [S1][S3][S5][S6].
On length tolerance, only IS 2048 gives a direct number: -0.2 mm for keys up to 28 mm, -0.3 mm for 32 to 80 mm, -0.3 mm for 90 mm and above, with matching positive tolerance on the keyway [S6]. ANSI B17.1 leaves length as application-driven, recommending roughly 1.5 x shaft diameter and relying on the keyseat alignment limits of 0.010 in offset and 0.002 in lead to police assembly geometry [S2][S5]. BS 46 gives a preferred length range, 3/4 in to 3 in for 1/4 in square stock, but does not pin a numeric length tolerance on the key itself in the imperial extract [S3]. For a 1.37 in blank, the practical write-up is: BS 46 size, ANSI B17.1 Class 2 side fit, IS 2048 length window 32 to 80 mm with -0.3 mm on the key and +0.3 mm on the keyway.
Procurement, Inspection, and Closing Checklist
For a 0.250 x 0.250 x 1.37 in square key, write the line as: "Square key, 0.250 x 0.250 x 1.37 in, to BS 46 Part 1:1958 size, ANSI B17.1 Class 2 side fit, length tolerance -0.012 in / +0.000 in per IS 2048:1983 (32 to 80 mm band), corner radius 0.010 in, keyway surface finish 250 microinches max." That single sentence covers width band, height band, side fit, length window, corner geometry, and finish in a form any machine shop or coupling supplier can quote against [S1][S2][S3][S4][S5][S6].
On receipt, check three things: key width with a go/no-go gage set to 0.250 to 0.252 in, key length with a caliper or micrometer against a 1.358 in minimum (1.37 in minus 0.012 in), and corner radius against a 0.010 in radius template. Verify on the drawing that the hub keyway length is on the plus side of the key length, because the IS 2048 keyway tolerance is +0.3 mm, not -0.3 mm, and a keyway cut short of the key will not seat [S6]. A misread here is the single most common field failure on small square keys, and it is fully preventable by writing the length tolerance the same way the standard does. The same discipline shows up in adjacent spec work, from V-belt selection in mining duty to the sensor side of the plant, where a pressure transmitter range code and a square key length code both succeed or fail on whether the tolerance band was written into the print.
Component reference pages worth checking: flow meter.