Metric and Imperial Units in the Bearings World – an Increment of Advice
Rexnord Aerospace products can be found in a variety of aircrafts, from civil to military, fixed to rotating wings. In today’s globalized world, it is not uncommon that an aircraft designed in imperial units ends up in a metric country.
Do you know the origin of the inch unit? Cutting a very long story short, it comes from the Old Engilsh ynce and was equivalent ot one twelfth of a Roman foot. During the 12nd century the King David of Scotland defined it as the average of the thumb breadth at the base of the nail of three men of different heights. Averaging the dimension of three men would give the measurement more consistency. The actual definition of the dimension evolved with time, until it has been officially defined as 2,54 cm back in 1959 as a result of a consensus of all the countries using imperial units.
If you search the web you will find several websites and mobile phone applications that can help you with metric to imperial conversion and vice versa. However, there are some tips that can be useful when you have an imperial part number at hands and want to discover what its dimension is.
1 – Increments of 1/16”
It is very common for imperial parts to have a couple digits that represent either their inner diameter or outside diameter. In imperial parts, the digits represent how many sixteenths of an inch the diameter is composed of. See the example below, coming from our P2A6500 series of pre-swaged, grease lubricated, split ball bearings.
In this case, the last two digits of the part number will provide to you how many sixteenths of an inch the ball inside diameter has got, in this case 4. So, in this case:
Using your preferred metric conversion calculator you will find out that this is equivalent to 6,35 mm.
2 – Increments of 1/32”
Although not as common as the 1/16 inch increments, some part numbers will also make use of 1/32 inch increments. That’s usually found in bushings and articles where the length of the part is also shown on the part number.
A good example is the Rexlon 2000 type III lined PT5M5BBXX Series of self-lubricated, flanged sleeve bearings, manufactured to SAE AS81934/2, previously MIL-B-81934/2:
Here, as we’ve discussed in the previous section, the first two digits highlighted “08” correspond to how many 1/16 inch increments the bushing ID has got. Therefore:
What would be the equivalent to 12,7 mm.
The next digits highlighted correspond to the bushing width. However, these represent how many 1/32 inch increments the width has got. Therefore the “12” you see there actually mean:
Which, if converted, equate to 9,525 mm.
Next time you have a bearing, bushing or seal part number in front of you, look for 1/16 and 1/32 multiples. If you do find them it is very likely they will help you find out at least one dimension of your part. However, if you want to find out more information about that part, please do not hesitate to send us an e-mail or call +1-805-583-5514.