By correctly interpreting which standard a PT component was made to, you can solve replacement specification problems and eliminate expensive special orders
The proliferation of foreign-made equipment in U.S. plants frequently spells trouble for those replacing shaft-mounted power transmission devices. When new couplings, sprockets, and belt pulleys bring with them unusually high costs, long lead times and recurring malfunctions, the cause often can be traced to misguided specification.
Here are two warning signs:
• Devices with non-standard bore and keyway dimensions. Such “specials” are not stocked by distributors or manufacturers. As a result, they cost significantly more than off-the-shelf models and may take many weeks to ship with some suppliers.
• Off-the-shelf models with the approximate dimensions of original parts. Improper fits, between an oversized bore and a shaft, for example, can lead to fretted bores and shafts, failed bearings, and other maintenance headaches.
These problems very often have the same root: ordering replacement parts with inch-based bore and keyway dimensions that match the old part, not knowing that the original part conformed to foreign standards. Fortunately, you can take simple steps to avoid common mistakes — and their respective costs and operational liabilities — when replacing shaft devices.
Apart from these steps, however, remember the importance of a thorough system analysis — including torque, operating speeds, horsepower, system inertia and nature of driven equipment — when specifying PT components.
Conquering world standards
Size and tolerance standards can be divided into two basic groups — metric and inch. Most of the industrialized world conforms to Germany’s DIN (Deutsche Industrial Normen) metric standards, and identical DIN-based systems (Japan’s JIS, ISO standards). Still used in the U.S. and U.K., inch-standard bore and keyway sizes differ significantly from metric standards.
Complicating matters, variations also exist within the basic metric and inch classifications. For example, British inch-based standards (BSI) are not the same as U.S. inch-based standards (AGMA). Some pre-1976 Japanese standards, used for machinery undoubtedly still in operation, differ from the metric standards to which Japan currently adheres. For equipment coupled to output shafts of electric motors, International Electrical Code (I.E.C.) metric standards and NEMA inch standards may apply.
When replacing shaft-mounted devices, use the following approach to get accurate results:
1. First, check the nameplates. Most often, the country of origin of the driving and driven machinery — gearboxes, motors, pumps, and compressors — suggests the correct standard. Consider both hubs of the shaft-mounted device separately, as one may be metric, the other inch.
2. Measure the original part. Use precise instruments and methods to measure bore diameter, keyway width, and dimension T of the part to be replaced (see box).
• Measuring in inch units. Standard Imperial (U.S., U.K.) bores and keyways are almost always designed to common inch and fractional-inch sizes (7/8, 1, 23/16, etc.). The measured bore size (in decimal equivalent) should be within the tolerance range of a nominal standard bore. If the decimal equivalents for all three measurements — bore diameter, keyway width, and keyway depth — do not match standard inch sizes plus accepted tolerances, the bore and keyway may be metric rather than an inch-based special.
• Measuring in metric units. With rare exception, nominal metric bore diameters are in whole millimeters (9, 14, 65, etc.). Measurements should fall within the tolerance range of the closest wholemillimeter bore. By referencing against the nominal bore size, you can easily determine standard metric keyway dimensions and compare them with component measurements for a match. (If measured in metrics, skip Step 4.)
3. Compare measured dimensions with current standards data. All inch and metric- based systems define a set of standard bore-keyway combinations for use with common shaft sizes. Only in rare cases do designers choose dimensions that deviate from those standards.
4. Convert to, or remeasure in, metric units. To convert with sufficient accuracy, multiply inches by 25.4001 (1 in. = 25.4001 mm). Then, repeating Step 3, compare with metric standards data.
If the measurements still don’t match up, the part probably is a special with non-standard bore and keyway dimensions. But, again, metric conversion very often turns up a standard foreign borekeyway combination that eliminates the need for a special part.
For example, a maintenance supervisor needs to replace a coupling for a mixer. The drive motor’s nameplate, though, is slightly damaged and its country of origin can’t be determined. But the coupling’s motor-side hub measures:
Bore = 1.1820 in. Keyway width = 0.3941 in. T dimension = 1.3133 in.
The bore doesn’t match a common inch size, Table 1, falling between the tolerance ranges for nominal bores of 11/8 (1.1250) and 13/16 (1.1875) in. Therefore, it appears to have a non-standard diameter. The keyway width, also obviously not an inch standard, may seem to confirm that an expensive special is needed.
However, following the outlined approach, it’s found that the original coupling is really a standard metric model:
Bore = 1.1820 in. x 25.4001 mm/in. = 30.0229 mm
Keyway width = 0.3941 in. x 25.4001 mm/in. = 10.0102 mm
T Dimension = 1.3133 in. x 25.4001 mm/in. = 33.3579 mm
For nominal bore diameters between 30 and 50 mm, Table 2, the metric bore tolerance data shows an allowable 20.000/10.025-mm variance*, within which the measured bore falls for the nearest whole millimeter (30.025 mm > 30.0229 mm > 30.000 mm).
Next, for bores between 30 and 38 mm, the metric keyway data, Table 3, shows a 10 mm x 8 mm (width-by-height) key as standard. The corresponding +O.0180 width tolerance comfortably includes the measured width (10.0180 mm > 10.0102 mm ƒ 9.9820 mm). The coupling's keyway depth (t2) must be calculated for comparison with the chart:
t2 = T– d = 33.3579 - 30.0229 = 3.3350 mm
Referenced against the tolerance data, the keyway depth of the old hub falls within the standard 3.3-3.5 mm range. A call to a distributor (or consulting a manufacturer’s catalog) reveals that this standard metric combination — 30-mm bore with a keyway for a 10 mm x 8 mm key — is quickly available at significantly lower cost than a special with the measured inch dimensions.
In another example, a pump’s nameplate reads “England” but doesn’t clearly state the year of manufacture. Again, starting with inch measurements, one coupling hub measures:
Bore = 2.8751 in. Keyway width = 0.7881 in. T dimension = 3.0738 in.
Because the bore is only 0.0001 in. above nominal, it obviously falls well within any of the tolerance ranges in Table 1. Therefore, the coupling appears at first to be standard. But the keyway dimensions rule that out, as the corresponding BSI tolerance ranges (W = 0.7500-0.7510; T = 3.2080-3.2140) don’t come close to including the measured keyway. Though the temptation may be to specify a standard bore with non-standard keyway, it’s highly unlikely that the coupling’s designer chose a 201/255-in. x 27/136-in. keyway. Again, before ordering a special (which can’t yet be ruled out), it’s best to try the conversion:
Bore = 2.8751 in. 325.4001 mm/in. = 73.0278 mm
Keyway width = 0.7881 in. x 25.4001 mm/in. = 20.0178 mm
T dimension = 3.0738 in. x 25.4001 mm/in. = 78.0748 mm
For nominal bore diameters between 50 and 80 mm, Table 2, the data shows that this coupling is within the allowable +0.030 mm variance for a 73-mm bore. The keyway depth is:
t2= T - d = 78.0748 - 73.0278 = 5.0470 mm
Checking against the keyway data, Table 3, shows that the width and depth match the metric standard within the given tolerances. Therefore, the coupling proves to be a standard — 73 mm bore with corresponding keyway for a 20 mm x 12 mm key.
Tap distributor expertise
As shown, this process will help you to better specify replacement couplings and other shaft-mounted devices. But, of course, special circumstances exist. One that particularly merits mention is the interference fit, wherein the hub’s bore diameter is less than the shaft’s diameter, perhaps by as much as 0.002 in. (0.05 mm) in large bores. If the original hub is hard to remove from the shaft (perhaps requiring heating), it may have been installed as an interference fit. On the other hand, galling between bore and shaft or chemical exposure may have “frozen” the two parts together.
A knowledgeable distributor can help solve such mysteries, and should be able to provide products with standard foreign bore-keyway combinations.
The tables shown are condensed. You can obtain a set of complete tables by writing to Lovejoy Inc., 2655 Wisconsin Ave., Downers Grove, Illinois, 60515; by fax request to (630) 852-2120; or via World Wide Web URL http://www.lovejoy-inc.com
* Tables 2 and 3 reflect the H7 International Tolerance grade for bores and JS9 grade for keyways, accuracy levels governed by the International Standards Organization (ISO), a consortium of the world’s metric-based standards bodies. H7 and JS9 are by far the most common tolerance grades for industrial equipment.
Ray Giegerich retired as Director of Engineering for Lovejoy Inc., Downers Grove, Illinois, and currently serves as a special consultant to the company.