Design Parameters For Sae 841 Bearings

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The following information will aid you in the proper selection of bearings to meet your needs. The sample pro- blems will illustrate the design criteria which must be established in order to select the proper bearing. Should any questions arise, our staff is ready to assist you.

Sample Design a bearing for 180
Problem: pound load at a speed of 1100 RPM

SHAFT SELECTION

Size Shaft size will be determined by the size and construction of the unit being designed. Refer to our PV Chart (page 42) in order to determine approx- mate criteria.

Example: A 1.000 shaft is chosen. The PV chart shows that a 1.000 shaft operating at 1100 RPM will carry 175 pounds per sq. inch load.

Materials Steel containing approx- imately 0.4% carbon are recommend- ed, and any steels with a lower con- tent should be avoided. The 1137-1141 series, for example, are ef- fective grades for shaft material. In addition, drill rod and hardened and ground steels can be used.

Hard chromium plated materials are recommended when corrosion resistance is required. The perfor- mance of stainless steel materials does not equal that of straight carbon- alloy steels. If stainless steel must be used, the 300 series should be avoid- ed. The 400 series is a good choice in stainless, and 416 stainless, heat- treated to maximum hardness, is the best.

Zinc or cadmium plating on shafts must be avoided as they are too soft and will ball up, resulting in a loss of bearing porosity, and lubrication.

Finishes Shaft finish is critical to long bearing life. Recommended finish is 16 RMS to 8 RMS. In some cases, a 32 RMS finish may be adequate if the application is not too precise.

BEARING WALL THICKNESS

For bearings less than 1/4 inch ID, the minimum wall thickness is 1/32 inch. For bearings 1/4 inch ID and larger, use the following formula to deter- mine wall thickness.

Example:
Wall thickness = .125x Shaft dia.
1" Shaft
Wall thickness = .125 x 1.000
Wall thickness = .125

Larger bearings with extremely thin walls are more costly to produce and result in increased unit costs.

BEARING LENGTH

Maximum length is determined by using a ratio of length to bore, not to exceed 4:1. For diameters less than 1/4 inch, use a smaller ratio.

To determine proper bearing length, refer to the PV chart (page 42). It will give the unit load per square for a specific speed and shaft size. By dividing the unit load per square inch into the total load to be carried by the bearing, the minimum length is found. In addition we recommend that a safety margin be used. By adding ex- tra length, the bearing avoids operating at extreme limits.

Example:

180 Ib. load
1100 RPM
1.000 Bearing ID

PV chart indicates
175 PSI UNIT LOAD

180 - 175 = 1.029 Minimum Length

1.029
.221
1.250

(Minimum Length)
(Safety margin)
Suggested length

LOADS AND SPEEDS

Load and speed limitations must be ascertained in order to insure proper bearing design. If design limits are ex- ceeded, the bearing life will decrease and could result in premature failure. To determine the capacity of a bear- ing, use the following PV formula:

P = The load per square inch of projected area
V = Shaft velocity in feet per minute
P =load
I.D. x Length
PV =P x V
1The maximum P is 2000 P.S.I.
2The maximum V is 1200 S.F.M.
3The maximum PV for R-Lube is 50,000

Example:

180 Ib. load
1100 RPM
1.000 Bearing ID x 1.250
Length bearing

P =180
1.000 x 1.250
P =144 WITHIN LIMIT
V =1100 x 1.00 x .262
V =288 SFM WITHIN LIMIT
PV =144 x 288
PV =41472 WITHIN LIMIT

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