Introduction

A shaft is a rotating member, usually of circular cross section, used to transmit power or motion. A nonrotating axle can readily be designed and analyzed as a static beam, rotating shafts are subject to fatigue loading. The deflection and slope analyses cannot be made until the geometry of the entire shaft has been defined. Thus deflection is a function of the geometry everywhere, whereas the stress at a section of interest is a function of local geometry. For this reason, shaft design allows a consideration of stress first. Then, after tentative values for the shaft dimensions have been established, the determination of the deflections and slopes can be made.

Shafts usually don't need to be surface hardened unless they serve as the actual journal of a bearing surface. Typical material choices for surface hardening include carburizing grades of ANSI 1020, 4320, 4820, and 8620.

Common tolerances

Commercially available shafts for rotary applications have straightness of 0.01/100 (mm/mm), 0.012 inch/ft, 0.003 inch/ft, circularity from 0.003 to 0.007 mm, perpendicularity of 0.05 mm and concentricity of 0.06 mm.

Bearings

Several companies have interactive catalogues to help you find the right size and type of bearing you need. These include SKF Bearings, NTN Bearings, RBC Bearings, ISB Bearing and AST Bearings.

Retaining Rings

The following form quickly verify design strength of the Ring and Groove of the internal and external types.

Sealing O-Rings

An O-ring seal is used to prevent the loss of a fluid or gas. The seal assembly consists of an elastomer O-ring and a gland. The rubber absorbs the stack-up of tolerances of the unit and its internal memory maintains the sealed condition. All robust seals are characterized by the absence of any pathway by which fluid or gas might escape. Detail differences exist in the manner by which zero clearance is obtained: welding, brazing, soldering, ground fits or lapped finishes; or the yielding of a softer material wholly or partially confined between two harder and stiffer members of the assembly. The O-ring seal falls in the latter class.

Design for Assembly

Figure- Proper designs for installation of O-rings.

Static - Aerospace Hydraulic Packing and Gasket: Design tables are available here that contain design dimensions for Male groove in pistons, and this one for Female groove in sleeve hubs (Table 4-1).

Static - Industrial: Design tables are available here that contain design dimensions for Male Groove in pistons and Female Groove in sleeve hubs (Table 4-2).

Dynamic - Aerospace Hydraulic Packing and Gasket (Per SAE AS4716): Design tables are available here that contain design dimensions for Male groove in pistons, and this one for Female groove in sleeve hubs (Table 5-1).

Industrial-Reciprocating Packing and Gasket: Design tables are available here that contain design dimensions for Male groove in pistons, and this one for Female groove in sleeve hubs (Table 5-2).

Parts Suppliers

  • Boston Gear is a good supplier of Gear Drives, Gear Heads, Clutches, Bearings, Variable Speed Drives, and others.
  • ISB Bearings is a good supplier of ball, needle, thrust bearings, bushes, linear bearings, etc.

Shaft Alignment
Basic

Tools needed: Safety glasses, straight-edge metal ruler, filler gauges, a caliper, shims, wrenches, dead-blow hammer.
Verify angular alignment: Use the caliper to measure the distance (FIG.1) between the hub faces at the 12, 3, 6, and 9 o'clock positions. If the distance difference between the 12 and 6 positions (vertical) are out of tolerance, use shims to try and correct this. If the same occurs between 3 and 9 (horizontal) use the hammer to correct this.
Verify parallel alignment: Use the straight-edge (FIG. 2) to check the the hubs align vertically (12-6 positions) and horizontally (3-9 positions). Use the filler gauge to measure any misalignments and correct this using the hammer and shims as previously described.
End procedure: Once the alignment is completed, tighten all fasteners holding the shafts in place to the recommended torque and install the flexible coupling connecting the hubs.
Using a Dial Indicator

Tools needed: Safety glasses, run out indicator, shims, wrenches, dead-blow hammer.
Verify alignment: Position the dial probe at the surface of the hub or rim of the coupler and perpendicular to the axis of the shaft (FIG. 1a-b); there one can check run out caused by eccentricity and bent shaft. By positioning the dial in the axial direction (FIG. 2a) we can also check how skewed the hub assembly is. Next, slowly rotate the shaft and note the points at which the needle reverse direction. Set the zero-line of the dial to one of these points. Next, rotate the shaft again to obtain the total run out reading.
To determine whether misalignment is due to bent shaft or eccentric hub: Position the dial probe on the surface of the shaft and as close to the hub as possible (FIG. 2b). If the run out at this location accounts for more than half the amount measured at the hub, then most of the error is caused by a bent shaft. Make corrections as needed.
Other Methods

Coupling Alignment
Basic
  1. Before Installing: Confirm that the clamping screws are loosened, and wipe clean the inner bore and shaft surfaces of dust and oils.
  2. Insert the shaft into the coupling while taking care not to apply excessive compressive/tensile forces on the disc section.
  3. Quick Lateral Misalignment Check: With all the locking screws loose, slide the coupling axially over both shafts (FIG. 1) to ensure smooth sliding movements. If not smooth, perform the lateral alignment procedure again.
  4. Quick Angular Misalignment Check: Rotate the coupling/shaft (FIG. 2) and visually check for smooth and even movement. If not smooth, perform the lateral alignment procedure again.
  5. Final Assembly: With a torque wrench, tighten the screws on both the motor and driven shafts to the recomended torques.
  6. Verify Operations: Check for tightening torques after approximately 30min. of continuous operation to account for any initial self-loosening of the screws
  7. For your safety, enclose the revolving parts of equipment with a protective cover.
  8. If the coupling item generates abnormal metallic noises, immediately turn off the equipment and identify the cause, such noises may be an indication of improper alignment, shaft interference or a loose screw.
  9. For applications with a large load fluctuation, apply an adhesive agent on the coupling's screws to prevent them from loosening, alternatively select 1 size larger couplings.