In our work on a specific segment of a continuous casting machine, we encountered a critical transmission component: the worm gear gearbox used in the adjusting and clamping mechanism. This gearbox, which governs the roll gap adjustment of the segment, is a typical worm gear pair combined with a trapezoidal thread lead screw. In this article, we present a detailed manufacturing process for the key parts of this worm gear gearbox, outlining the difficulties, solutions, and verification methods that ensure the transmission accuracy and reliability of the worm gear system.
The segment is a horizontal section that supports the cast strand for further cooling until complete solidification. The mechanical part consists of inner and outer arc frames, movable beams, free roll assemblies, driven roll assemblies, adjusting devices, and guide wheel assemblies. The worm gear gearbox is the transmission element of the adjusting device. It is connected to the lower frame via a trapezoidal thread tie rod. The roll gap adjustment of the segment is realized by the lifting and lowering of four tie rods, which are driven by an external motor through the worm gear gearbox, converting rotary motion into linear motion via the trapezoidal thread.
The worm gear gearbox structure is as follows: the worm is connected to the external drive and drives the worm gear. Both ends of the worm are supported by single-row tapered roller bearings (type 30211) and fixed on the housing through covers. The upper part of the worm gear is a flange that is fixed on the housing, while the lower part is supported by a thrust ball bearing (type 51156) and a locating sleeve. The thrust bearing is mounted on the housing, and the locating sleeve is assembled into the housing through a bronze bushing on its outer cylindrical surface. Inside the locating sleeve, a threaded sleeve is installed, and the adjustment is fixed by external teeth on the threaded sleeve and a tooth plate. Both the worm and the threaded sleeve have trapezoidal threads that engage with the tie rod.
The manufacturing difficulties of the worm gear gearbox components are summarized in the following table, along with the material and critical features.
| Component | Material | Critical Features / Challenges |
|---|---|---|
| Tie Rod (拉杆) | 45 steel, Class IV forging | Trapezoidal thread (Tr180×20-9c) with high precision; M140×4 thread on right side; hard chrome plating on middle section (diameter φ220e8); stringent dimensional and surface finish requirements. |
| Housing (壳体) | ZG230-450 cast steel | High machining precision for bearing bores (φ305J7, φ100J7); center distance 200±0.036 mm between worm and worm gear axes; bronze bushing interference fit. |
| Threaded Sleeve (螺纹套) | Steel (likely 45 steel) | External teeth on left end for locking with tooth plate; internal trapezoidal thread (Tr180×20) matching tie rod; concentricity and thread profile. |
| Worm (蜗杆) | 20CrMnTi, carburized steel | Carburized case depth 1+0.2 mm, hardness 60±0.2 HRC; gear tooth grinding after heat treatment; center distance and meshing accuracy with worm gear; thread quality. |
| Worm Gear (蜗轮) | ZCuAl10Fe3 cast copper alloy | Accurate center distance; internal trapezoidal thread (Tr180×20) for tie rod engagement; meshing with worm; contact pattern requirements. |
| Gearbox Assembly | — | Simultaneously control of trapezoidal thread fit and worm gear mesh; bidirectional adjustment; precise assembly sequence. |
To overcome these challenges, we developed detailed manufacturing solutions for each component. The following sections describe the key steps and control measures.
Manufacturing of the Tie Rod
The tie rod is a critical load-bearing part. Its manufacturing process includes rough turning, ultrasonic testing (UT per DIN EN 10228-3 grade 2), quenching and tempering to 242-287 HB, semi-finish turning, cylindrical grinding, dye penetrant inspection, thread cutting, and hard chrome plating. Special attention was given to the trapezoidal thread Tr180×20-9c. After external cylindrical grinding of the reference diameter φ180h9, the thread was cut using the following inspection method: a tooth thickness caliper measured the pitch diameter tooth thickness to be 9.70-0.1 mm, and a profile template (see figure in original, but here we omit the figure reference) was used to check the tooth form. The thread external diameter φ180h9 was used as the locating surface for both measurements. The hard chrome plating was applied to the φ220e8 area, with two test coupons (10×20×20 mm, 45 steel) plated simultaneously to verify hardness 950-1050 HV. During transportation, the tie rod was supported on V-blocks to prevent deformation.
Manufacturing of the Housing
The housing is a cast steel part that supports the entire worm gear gearbox. The machining process was divided into rough and finish operations. The bronze bushing (for the locating sleeve) was assembled using liquid nitrogen cooling at -70°C for 2 hours, then allowed to return to room temperature before finish machining. The critical center distance between the worm bore (φ100J7) and the worm gear bore (φ305J7) was maintained at 200±0.036 mm. The following table summarizes the key tolerances:
| Feature | Specification | Method |
|---|---|---|
| Worm gear bore | φ305J7 | Boring on CNC machine, in one setup together with other bores |
| Worm bore | φ100J7 | Boring after rotating the workpiece 90° |
| Center distance | 200±0.036 mm | Coordinate measurement; verified by machining in same clamping |
| Bronze bushing interference | Press fit (calculated based on material) | Liquid nitrogen cooling assembly |
Manufacturing of the Threaded Sleeve
The threaded sleeve has an internal trapezoidal thread Tr180×20 that matches the tie rod, and external teeth at the left end for locking with the tooth plate. The internal thread was cut using a lathe, and its pitch diameter tooth thickness was controlled to 9.750-0.1 mm, with tooth height 4.2-4.4 mm, measured with a tooth thickness caliper using the internal diameter φ160H7 as the reference. A profile template (similar to the tie rod) was also used. After threading, the external teeth were cut in coordination with the tooth plate for final assembly.
Manufacturing of the Worm
The worm is carburized and hardened. The manufacturing sequence was:
- Rough turning with extra length (25 mm at each end) for center holes (DIN332-DS M16).
- Grinding the tooth tip diameter to size, plus two 30-50 mm locating bands on the shaft journals for gear cutting.
- Gear hobbing, leaving 0.3-0.4 mm allowance on tooth flanks for grinding.
- Carburizing and hardening: case depth 1+0.2 mm, hardness 60±0.2 HRC.
- Re-machining center holes on a lathe.
- Grinding all shaft journal diameters to final size.
- Gear grinding: the worm was mounted between centers with radial runout ≤0.003 mm; the workpiece radial runout after clamping ≤0.02 mm. After grinding, the worm was paired with the mating worm gear to check the center distance (200±0.036 mm). The contact pattern was inspected: along tooth height ≥55%, along length ≥50%, with the pattern biased toward the exit side, and no edge contact at the tooth tip or ends.
- Cutting the keyway.
The worm gear meshing specifications are summarized below:
| Parameter | Requirement |
|---|---|
| Center distance (with mating worm gear) | 200±0.036 mm |
| Tooth contact – height | ≥55% |
| Tooth contact – length | ≥50% |
| Position of contact pattern | Biased toward exit side; no edge contact |
| Backlash (worm gear pair) | 0.03–0.05 mm |
Manufacturing of the Worm Gear
The worm gear is made of cast copper alloy ZCuAl10Fe3. The process included rough and finish turning, drilling and tapping, gear hobbing, and final turning of the internal trapezoidal thread. During gear hobbing, the workpiece was clamped with radial runout ≤0.002 mm on the bore and ≤0.003 mm on the tooth tip diameter. After hobbing, the worm gear was paired with the worm to check meshing. The internal trapezoidal thread Tr180×20 was cut last, with the following inspection: pitch diameter tooth thickness 9.50-0.1 mm, tooth height 4.96-5.0 mm, using the bore φ160H7 as reference. The worm gear thread profile was also verified with a template.
Assembly of the Worm Gear Gearbox
The assembly process required careful control of both the trapezoidal thread fit and the worm gear meshing. The steps were:
- Pre-inspection of all components: check trapezoidal threads of tie rod, threaded sleeve, and worm gear with 0.05 mm feeler gauge (no-go). Verify dimensions and tolerances.
- Mount tapered roller bearings (30211) on the worm shaft, with inner ring fully seated (gap ≤0.05 mm). Mark match pairs.
- Install thrust ball bearing (51156) into housing: loose ring on housing, tight ring on worm gear. Place worm into housing, then match the outer bearing rings. Install end covers and seals. Adjust axial clearance of tapered bearings to 0.05-0.1 mm by machining the cover face; check with dial indicator (outer ring axial movement 0.1-0.2 mm).
- Install worm gear into housing. Use shims between thrust bearing and housing to align the worm gear centerline with the worm centerline. Manually rotate the worm gear and check with a dial indicator: the indicator reading on the housing face should fluctuate less than 0.025 mm per revolution. Then check meshing: contact pattern along height ≥55%, length ≥50%, biased to exit side. If not acceptable, adjust shims.
- After confirming meshing, install O-rings on worm gear and flange. Place a 0.2 mm shim between the flange and worm gear. Mount the flange using temporary shims (process shims). After final testing, remove the temporary shims and replace with ground shims of the calculated thickness.
- Flip the gearbox. Install the seal on the locating sleeve, then install the locating sleeve into the housing with countersunk hex bolts (using Loctite for locking).
- Place the gearbox on a flat plate (flatness ≤0.05 mm). Use a hydraulic jack (≥60 kN) to push the locating sleeve upward to eliminate thrust bearing clearance. Measure the axial displacement with three dial indicators equally spaced (120°). Apply the jack three times, rotating 120° each time. The 9 readings (3 indicators × 3 pushes) should be within 0.1-0.2 mm, with max-min difference ≤0.03 mm. Calculate the average displacement a. The required shim thickness H is given by:
$$ H = B + a – 0.13 $$
where B is the actual thickness of the process shim used, and 0.13 mm is the ideal worm gear axial clearance. Adjust the process shim thickness accordingly.
- Install the tie rod. Screw it in until it protrudes 190 mm below the housing lower face. Assemble the threaded sleeve onto the locating sleeve. Tighten the threaded sleeve with a torque of 300 N·m using a torque wrench and a special adapter that connects to the tooth plate. Then weld a nut onto the adapter for later removal.
- Adjust the clearance between the threaded sleeve and the locating sleeve to 0.17 mm. Calculate based on actual dimensions: tooth plate thickness, locating sleeve height, threaded sleeve length. Tighten the threaded sleeve, then rotate it back half a turn to achieve the 0.17 mm gap. Lock the position with the tooth plate.
- Repeat the dial indicator measurement as in step 7, but now measure the clearance b at 9 points (as shown in figure, but omitted in text). All 9 readings should be between 0.1 and 0.17 mm, with max-min ≤0.03 mm. Record the average b. For the four gearboxes on one segment, the b values must differ by ≤0.04 mm. Before final assembly of the segment, disassemble the tooth plate, threaded sleeve, and tie rod for transport.
The following table summarizes the critical assembly parameters and formulas used:
| Parameter | Symbol / Value | Formula or Method |
|---|---|---|
| Worm bearing axial clearance | 0.05–0.1 mm | Measured by axial movement of outer ring |
| Worm gear axial runout (per rev) | < 0.025 mm | Dial indicator on housing face |
| Worm gear meshing contact | Height ≥55%, Length ≥50% | Visual with marking compound |
| Worm gear pair backlash | 0.03–0.05 mm | Dial indicator at worm gear periphery |
| Thrust bearing clearance elimination | 0.1–0.2 mm (displacement a) | Jack and 3 dial indicators; average a |
| Shim thickness H | mm | $$ H = B + a – 0.13 $$ |
| Threaded sleeve gap | 0.17 mm | Calculated from actual dimensions; verified with feeler gauge |
| Uniformity among 4 gearboxes | Max difference in b ≤ 0.04 mm | Compare average b values |
Throughout the manufacturing and assembly of this worm gear gearbox, we followed strict process controls and inspection protocols. The result was a smooth-running, high-precision worm gear transmission that met all performance requirements on the first attempt. The detailed procedures described here provide a solid foundation for future production of similar worm gear drives, ensuring consistent quality and reliability.