Chassis Lubrication and Driveline Inspection
Objective. Students will grease all chassis fittings on a Class 7–8 truck using the OEM grease specification and a calibrated grease gun, inspect U-joints, slip yokes, and fifth-wheel plate for wear and lubrication, and document all findings on the PM inspection form.
This lesson, its worked examples and its practice questions has not yet been verified by a certified examiner or master technician in the trade or by a certification or approval body. It is a study draft and may contain errors. Do not use it as instructions for work on a job site or on live equipment. Always follow your instructor, your supervising professional, and current official references and regulations.
VoTechWorks program approval and accreditation are still in progress. Credentials are issued only by the certifying bodies named in each program, after their own exams.
Why Chassis Lubrication Matters
Grease is the only thing standing between metal-to-metal contact at dozens of pivot and wear points on a Class 7–8 truck. When a king pin runs dry, it scores in hours. When a fifth-wheel plate lacks grease, the trailer kingpin and locking jaws wear prematurely, and coupling geometry drifts out of tolerance. A disciplined, documented PM lube service is one of the highest-value tasks a diesel technician performs, and it directly affects vehicle safety, tire wear, and steering precision.
Safety First: LOTO and PPE
Before you touch any fitting on a truck that is raised, you must apply lockout/tagout (LOTO) per your shop's energy-control procedure. Block the drive wheels, place the transmission in park or neutral with the parking brake set, raise the vehicle on approved jack stands or position it over the pit, then tag and lock the ignition. Never work under a vehicle supported only by a floor jack. Wear chemical-resistant gloves because petroleum-based greases can cause skin sensitization with repeated exposure. Safety glasses protect against grease purging unexpectedly from a blown boot.
Reading the OEM Chassis Lube Chart
Every manufacturer publishes a chassis lubrication chart that lists every grease fitting location, the required NLGI grade, and the service interval. Common locations on a Class 7–8 truck include:
- King pins and king pin thrust bearings – steer axle, one or two fittings per knuckle
- Tie rod ends and drag link ends – quantity varies by axle configuration
- Slack adjuster pins – one fitting per brake assembly
- Spring pins (front and rear) – eye bolts and center pins on leaf-spring suspensions
- Fifth-wheel pivot pin and slide mechanism – on sliding fifth wheels, also grease the slide rails
- Driveshaft U-joints – most have a single cross-fitting per joint
- Slip yokes – one fitting per telescoping section of the driveshaft
Before approaching the truck, mark every fitting on your truck diagram. This pre-walk step prevents missed locations and helps you plan an efficient path around the vehicle.
Grease Selection: NLGI Grade and Type
NLGI (National Lubricating Grease Institute) grades describe consistency, from 000 (nearly fluid) to 6 (very firm). Most heavy-truck chassis fittings call for NLGI Grade 1 or 2 multipurpose grease, often with an EP (extreme pressure) additive. The OEM chart is the authority; substituting a lighter or heavier grade, or omitting EP additives on U-joints, reduces film strength. Load the grease gun with the correct cartridge, purge air from the coupler, and confirm the gun is calibrated so you know roughly how many pumps deliver one ounce.
Greasing Technique
1. Clean the fitting with a rag before attaching the coupler to prevent pumping dirt into the joint.
2. Attach the coupler firmly so it seals; a loose connection forces grease out around the coupler, not into the joint.
3. Pump slowly until fresh grease purges from the seal or boot — this confirms the old grease has been displaced. Stop when fresh grease appears; over-packing can blow out lip seals.
4. Exception — sealed bearings: Some wheel-end and U-joints are factory-sealed with no fitting. Do not drill and add a fitting unless the OEM authorizes it.
5. Clean excess grease from around the boot and note any torn boots or missing dust caps on the PM form — they are failure items.
U-Joint Inspection
With the driveshaft accessible, grasp the shaft on each side of the joint and attempt to move it radially (side to side) and axially (end to end). Any perceptible radial play at the cross indicates worn needle bearings. Axial movement points to worn snap-ring grooves or bearing cups walking out. Either condition is a replacement trigger — a failed U-joint at highway speed can drop the driveshaft, puncture the fuel tank, or cause loss of vehicle control. Check that the cross-fitting accepts grease without bypassing; a plugged fitting means the joint must be replaced or the cross cleaned and re-fitted per OEM guidance.
Slip-Yoke Inspection
The slip yoke allows driveshaft length to change as the suspension articulates. Inspect splines for corrosion (rust or pitting that can bind and create vibration), missing grease, or spline wear (measured as circumferential lash). Grease the yoke fitting until a thin film of fresh grease appears at the yoke seal; do not over-pump, which can unseat the seal.
Fifth-Wheel Inspection
The fifth wheel is the primary coupling between tractor and trailer. Inspect:
- Locking jaws (kingpin locking mechanism): With the trailer uncoupled, manually operate the release handle and visually confirm jaw movement. Jaws must fully close around a test kingpin without excess side play. Check OEM specs for jaw wear limits — worn jaws allow trailer movement that generates fatigue cracking in the coupling plate.
- Fifth-wheel plate: Should have a continuous, even film of fifth-wheel grease (typically an NLGI 1 or 2 product formulated to resist wash-off). Bare metal or heavy scoring is a deficiency.
- Pivot pin and mounting bolts: Verify the pivot pin moves freely and mounting hardware is torqued to spec.
Documenting on the PM Form
Every finding — grease purged or not, boot condition, play measurements, fifth-wheel jaw condition — must be recorded. A blank field on a PM form is as unacceptable as a missed fitting. Mark each item Pass, Fail, or Needs Monitoring with a brief note. Failed safety-critical items (dry king pins, worn U-joints, cracked fifth-wheel jaw) must be escalated before the vehicle returns to service. Your instructor will review the form as the lab completion check.
Key terms10
Worked examples4
- Identify the delivery rate: 1 oz per 40 pumps, which equals 0.025 oz per pump.
- Determine the required quantity: 0.5 oz.
- Divide required quantity by delivery per pump: 0.5 oz ÷ 0.025 oz/pump = 20 pumps.
- Calculate grease for U-joints: 6 fittings × 0.5 oz = 3.0 oz.
- Calculate grease for slip yokes: 2 fittings × 0.75 oz = 1.5 oz.
- Total grease used: 3.0 oz + 1.5 oz = 4.5 oz.
- Grease remaining: 14.0 oz − 4.5 oz = 9.5 oz.
- Calculate total fitting time: 17 fittings × 45 seconds = 765 seconds.
- Convert the allowance to seconds: 15 minutes × 60 seconds/minute = 900 seconds.
- Compare: 765 seconds used vs. 900 seconds allowed; 900 − 765 = 135 seconds (2 min 15 sec) of margin.
- Identify measured play: 0.08 in.
- Identify OEM limit: 0.03 in.
- Calculate excess: 0.08 in − 0.03 in = 0.05 in over the limit.
- Because the measurement exceeds the OEM wear limit, the U-joint must be replaced before the vehicle returns to service — grease alone will not correct worn needle bearings.
The math in this lesson2
Materials: Class 8 shop truck on pit or jack stands, grease gun with correct NLGI-grade grease, grease fittings (assorted), torque wrench, LOTO kit, PM inspection form, OEM chassis lube chart
Check: Completed PM inspection form reviewed by instructor: all fittings located and greased, U-joint and fifth-wheel inspection findings documented with pass/fail against OEM wear limits; missing any safety-critical item (e.g., king pin not greased) = task repeated.
Practice check8
Answer each question, then check it. Nothing is recorded; this is your own practice.
