Fasteners, joints, and bearings

• Explain bolt preload, joint friction, and why tightening torque is only an indirect control variable.
• Choose and align a simple bushing or bearing for radial and axial load paths.

• forces-free-body
• torque-gearing

A screw does more than pin two parts together. Tightening stretches the screw slightly and compresses the joint, creating preload. External sideways load can then be resisted by friction between clamped faces instead of repeatedly shearing the screw. A loose printer frame rattles because contact opens and slips; simply installing a larger screw may not fix poor seating, weak material, or absent locking.

An approximate relationship is T ≈ KFd, where tightening torque T produces preload F in a screw of nominal diameter d and K lumps thread and under-head friction. K varies substantially with finish and lubrication, so torque is an imprecise proxy for preload. A washer spreads contact pressure; a locknut, thread-forming feature, or suitable threadlocker can resist loosening, but none repairs stripped threads. In soft printed plastic, a through-bolt with washer or a correctly installed insert may be more repeatable than repeatedly threading into the polymer.

Bearings constrain motion while carrying loads. Radial load acts across a shaft; axial or thrust load acts along it. Plain bushings tolerate dirt and can be simple, while rolling bearings reduce friction but need appropriate fits and alignment. Two bearings forced into misaligned holes can bind because each imposes a slightly different shaft axis. The build decision is therefore joint and load-path specific: place supports apart enough to resist moment, provide a shoulder or separate thrust surface when axial force exists, and avoid over-constraining an imperfect frame.

A common misconception is that tighter is always safer. Excess torque can yield a small screw, crush a screen bezel, pull an insert from plastic, or overload a bearing. Conversely, finger-tight hardware can lose preload under vibration. Another mistake is pressing force through rolling elements during installation; load should be applied to the ring being fitted. For novice builds, use published hardware guidance when available and validate low-consequence joints experimentally.

Problem

Estimate the preload from 1.2 N·m tightening torque on an M4 screw using K = 0.20, then estimate friction capacity at the single interface between two mating clamped faces with μ = 0.25.

Steps

1. Use d = 4.0 mm = 0.0040 m and F = T/(Kd).
2. F = 1.2 N·m ÷ (0.20 × 0.0040 m) = 1500 N estimated preload.
3. With one effective slip plane, friction capacity is μF = 0.25 × 1500 N = 375 N.

Result

The rough model predicts 1.5 kN preload and 375 N slip resistance.

Sanity check

The large sensitivity to K matters: if K were 0.30, estimated preload would fall to 1000 N. This is not a recommended torque or proof the materials can withstand it.

A shaft spins freely in either bearing alone but binds when both bearings are bolted rigidly to a frame. What is the best first diagnosis?

A. The motor needs more voltage B. The bearing bores are misaligned C. The shaft needs a higher gear ratio D. The bolts are too strong

Your answer: ______________________________

Reasoning:

Goal

Observe preload loss and bearing alignment in a low-load linkage pivot.

Equipment

• Cardboard or printed linkage strips
• M4 bolt, washers, and locknut
• Drinking-straw bushing
• Small spring scale

Procedure

1. Build a pivot first with the bolt clamping directly on the moving strips; note the tightening range between wobble and binding.
2. Add a straw bushing cut slightly longer than the moving stack so the bolt clamps the bushing while the link rotates around it.
3. Predict and then measure approximate pull force needed to start motion in both versions.
4. Deliberate fault: skew one support washer or bracket; diagnose alignment by rotating slowly and observing whether resistance varies with angle.

What to observe

• Expected: a correctly sized bushing separates clamp preload from pivot clearance and widens the usable tightening range.
• Measure: actual start force and side play; record variation rather than claiming a universal friction coefficient.

Transfer challenge

Specify the fastener, washer, bushing, and axial restraint for one desk-robot wheel or linkage joint.

Cautions

• Deburr cut tubing and keep the low-load test away from pinch points.

Learner: ____________________ Date: ____________________

Evidence artifact or file: ____________________________________________

Notes: