S-Cam Brake Parts Inspection/Replacement Criteria

When performing S-cam brake service, it is important to understand the serviceability of the individual components that comprise an S-cam assembly. The following information will help determine whether an individual component may be reused or should be replaced.

The following components should be replaced whenever an S-cam brake is relined.

1. Return springs.
2. Anchor pin springs.
3. Roller retainers.
4. Anchor pin bushings (dual anchor pin design).
5. Cam rollers.

1. Brake imbalance or "pull."
2. Increased stopping distance.
3. Abnormal lining or drum wear.
4. Brakes not adjusting correctly.
5. Unusual vibration or noise.

This list is not all-inclusive. Other performance factors may be impacted as well. The following inspection criteria should be used when determining if a component should be replaced.

• Brake shoe and lining assembly  - At all times if a reline is required, reline both wheel ends of an axle. Both wheel ends on an axle must use the same type and manufacturer of lining. See Fig 1 for sample edge code used to identify lining. For information regarding drum conditions and serviceability, refer to RP 608B BRAKE DRUMS AND ROTORS.

  When doing brake reline work it is imperative that key identifiers on replaced shoes be the same. To determine when linings should be replaced due to normal wear, refer to the Commercial Vehicle Safety Alliance (CVSA) North American Uniform Out-Of-Service Criteria  . The

  Fig 1: Sample Edge Code - EES 600 FF

  

  EES 600 FF is the identifier for this brake block. All other information is relevant to (1) the date block was made, (2) the plant, or (3) the manufacturer.

  Fig 2: Sample Edge Code - Blue-XLS FF

  

  Blue-XLS FF is the identifier of this brake block (left).

  following is part of the CVSA criteria: "Lining with a thickness less than 1/4" or to wear indicator if so marked, measured at the shoe center." See Fig 3 for examples of common wear indicators.

  Fig 3: Checking Brake Shoe For Wear Indicators

  

  If a vehicle is found to have lining worn to the above criteria it is to be put out of service. TMC recommends that linings be replaced before the above minimums are met. (Fig 4 shows a lining exhibiting normal wear. Note even wear of lining on shoe.)

  Fig 4: Checking Brake Shoe For Lining Exhibiting Normal Wear

  
• Replacement Shoe and Lining Assembly Inspection  - When installing relined shoes inspect the following:
  1. Edge codes. All edge codes should match (see TMC RP 606B, BRAKE LINING PROCEDURES ).
  2. Rivets. All rivets are installed and tight.
  3. Welds. Inspect all welds for breaks.
  4. Linings. Broken, cracked lining, gaps between lining and shoe.
  5. Anchor pin and roller seats. Look for wear, out of round or flaring (see TMC RP 605B, RECONDITIONING BRAKE SHOES for details).
• Contaminated Lining  - When the lining material is oil soaked, it should be replaced. It is not recommended to "clean" and reuse an oil contaminated shoe and lining assembly. Follow the guidelines above and reline all wheel ends on the axle(s). (See Fig 5)
  Fig 5: Checking Brake Shoe For Oil Soaked Lining

  
• Camshaft Seals  - If any evidence exists of grease leaking into the drum area, the seals must be replaced.
• Camshaft Bushings  - Camshaft radial play must be checked. Excessive camshaft movement either up and down or side-to-side will effect brake performance. To achieve an accurate measurement, disconnect the brake adjuster clevis pin from the chamber push rod. Check radial play at both ends of the camshaft using a dial indicator (see Fig 6). Refer to brake manufacturers bulletins and/or maintenance manuals for specifications. When in doubt, replace.
  Fig 6: Checking Radial Play At Both Ends Of Camshaft Using Dial Indicator

  
  Fig 7: Checking Cam Journals For Wear

  
• Camshafts  - Camshaft replacement is required if the following is evident (see Fig 7):
  1. Wear of the bushing journals on the camshaft has led to grooves.
  2. Groove at seal contact area on cam journal.
  3. Corrosion resulting in pitting on bushing journals.
  4. Inspection of the S-cam end of shaft shows any galling, flat spots, corrosion.
  5. Spline end wear, corrosion, snap ring groove wear.
• Normal camshaft wear criteria  -
  1. Bushing journals exhibit polishing with no evidence of grooves or corrosion.
  2. When checking journal for seal wear, a slight polishing at seal contact area may be evident.
  3. S-cam head normal wear will show polishing where roller has tracked.
  4. Spline wear is normal when a slight polishing of the splines is evident.
  5. Snap-ring groove exhibits polishing with no evidence of grooves, deformation, or corrosion.

  Tooling is commercially available to check critical dimensions. When in doubt, replace.

• Anchor pins  - There are two types of anchor pins in use: removable and fixed. If the anchor pins are removable, TMC recommends that the anchor pins be replaced at every brake reline. For fixed pin service refer to the brake manufacturers maintenance manual for further details and instructions.
• Spider  - Brake spider mounting bolts vary in size and specification. Some bolts have specialized plating or coating required for adequate performance. Refer to the vehicle manufacturer's recommendations for selecting replacement bolts. Tighten fasteners according to the manufacturer's recommended torque. It is imperative that the spider be securely fastened to the mounting flange.
• Cracks  - Thoroughly inspect brake spiders for cracks with careful attention to areas around the mounting holes, anchor pin bosses, cam bracket mounting bolts and transitional radii of the casting or stamping. If cracks are found replace  the spider. In cases where the spider is welded to the axle (such as trailer applications), contact the axle manufacturer for repair recommendations.
• Spider Anchor Pin Holes  - On foundation brakes that utilize a removable anchor pin, (with the bushing removed from the pin hole), inspect the anchor pin holes for:
  1. Eccentric wear. (See Fig 8)
  Fig 8: Checking Anchor Pin Hole
  
  2. Excessive wear (tools are available to check the hole size).

  If any of the above conditions exist, replace the spider.

• Spider Alignment  - If removed shoes show a non-concentric wear pattern, it is possible that the bolt torque is incorrect, or the spider is misaligned/bent. To ensure proper alignment of the spider to the centerline of the axle, use of a gauge is recommended. (See Fig 9)
  Fig 9: Checking Spider Alignment

  
• Air Chamber Mounting Brackets  - There are two types of chamber mounting brackets on S-cam brake systems. One style has the "wing" bracket welded to the cam tube (see Fig 10). On most trailer axles, the chamber mounting bracket is welded to the axle housing and there is no cam tube (see Fig 11).
  Fig 10: Locating Bracket Wing, Bracket Tube And Bracket Flange

  
  Fig 11: Locating Cam Head Bushing, Camshaft, Air Chamber Bracket And Chamber Bushing Or Support

  

  In either case, check for bends, twists, cracks, or other damage to the bracket. Also check air chamber mounting bolts for proper torque (reference air chamber manufacturer's recommendations). Loose bolts can lead to elongated mounting holes in the bracket. Bracket replacement is required if any of the above damage or conditions are found. In the case of trailer axles where the bracket is welded to the axle housing, contact the axle manufacturer for recommendations on the repair.

  Inspect all bracket mounting bolts for signs of looseness. Proper torque of these bolts is critical to brake performance. Tighten fasteners according to manufacturer's recommended torque.

  Loose bolts can lead to cracks in the bracket or spider. If looseness has caused elongated wear to the mounting holes, replace the assembly. For cast spiders, threads in the spider can be damaged by low bolt torque. Stripped holes in a cast spider requires spider replacement. Fasteners used for bracket attachment must meet the brake manufacturer's specifications and be tightened to their recommended torque.

  Inspect flange, tube, and welds for bends, twists, or cracks. Replace bracket if damage is found. In some cases, an external cam bracket support is used. These external supports are usually either welded to the axle housing or bolted to a suspension mounting bracket (see Fig 12 for examples). Inspect any external supports for damage such as cracks, bends, twists, or loose fasteners. Replace any damaged bracket supports. For those that are welded to the axle housing, contact the OEM for repair recommendations.

  Fig 12: Checking Bracket For Damage

  
• Dust Shields  - Dust shields are an option on most foundation brake assemblies. If the vehicle is equipped with dust shields look for:

1. Mounting bolt torque. If loose torque to proper spec. Prolonged looseness will lead to egg shaped holes.
2. Damage/dents.
3. Interference between the shield and the drums/lining.

If any of these conditions are evident, replace the dust shield.