A set screw does the least glamorous job in a machine — holding a pulley, gear or bearing collar on a shaft — and causes a disproportionate share of service calls when it works loose. Almost every failure comes from picking the wrong point type or under-specifying the locking method.
1. Four point types and what each is for
| Point | Standard | What it does | Use when |
|---|---|---|---|
| Cup point | GB/T 79 · DIN 916 | Ring edge bites into the shaft | General purpose; highest holding power on hardened shafts |
| Cone point | GB/T 78 · DIN 914 | Sharp tip penetrates deeply | Permanent location; thin shafts; spot-facing not possible |
| Flat point | GB/T 77 · DIN 913 | Flat contact, minimal damage | Frequent adjustment, soft shafts, flat already machined |
| Dog point | GB/T 80 · DIN 915 | Cylindrical pilot enters a hole | Positive location; shaft has a mating drilled spot |
2. Sizes, drive and material
- Metric range commonly stocked: M3–M20, lengths 3–60 mm; UNC/UNF inch sizes on request.
- Drive: hex socket (most common), slotted (older equipment), or Torx for automated assembly.
- Material: alloy steel 45H / 40Cr with black oxide for general use; A2-70 or A4-70 stainless where wash-down or corrosion matters.
- Hardness matters: a soft set screw deforms and loses grip. Specify hardened (HRC 45 min on the point) for shaft locking.
3. Why set screws back out — and six fixes
- Vibration + reverse rotation — the most common cause on reversing drives.
- Under-torqued assembly — "tight by feel" is typically 40 % of the required torque.
- Soft shaft material — the point brinells the shaft, preload drops, then it walks out.
| Method | Effectiveness | Comment |
|---|---|---|
| Nylon-patch screw | Good | Reusable a few times; simple upgrade |
| Threadlocker (medium strength) | Good | Clean threads before applying; serviceable with hand tools |
| Two screws at 90° | Better | Second screw locks the first; standard on collars |
| Flat on shaft (machined) | Best for adjustment | Flat point against a machined flat — no shaft damage |
| Key / keyway | Positive drive | Use for real torque transmission, not just location |
| Locking collar or clamp hub | Best overall | When the joint is truly critical |
Rule: a set screw locates; it should not transmit significant torque. If a coupling or pulley is slipping, the fix is a key, a clamp hub or an interference fit — not a bigger set screw.
4. Practical assembly notes
- Deburr the shaft and, where possible, provide a shallow flat or spot — it doubles reliability.
- Use the correct key size; a worn hex key rounds the socket and the screw is then unremovable.
- On stainless shafts, use a stainless set screw or expect galvanic corrosion; add anti-seize to prevent galling.