Inspection and repair on SEISA DBP 196-V gearbox

A visual inspection of the gearbox with the help of a videoscope was requested by the customer to identify the root cause of the excessive amount of metal and brass particles found on the bottom of the gearbox.
Two out of three planet wheel bearings were found badly damaged. Damage on the sun pinion was found as well, most likely as a result of the damaged planet bearings.

Based on the previously described finding of the visual inspection it was decided to replace all rolling element bearing as soon as possible. Gear damage was already started and was expected to progress quickly. Rolling element bearings were available on-site. Other replacement parts, such as the thrust blocks, a white metal radial bearing and a lantern ring were ordered at MHI (Seisa), but would only arrive in April 2012 at the earliest. The overhaul started by removing the inching drive and main drive electro motor. The mill’s grinding table was then loosened from the gearbox and lifted to enable the gearbox to slide from underneath. The gearbox was positioned for repair were normally the inching drive was located.

The gearbox consist of 4 major assembled components.
1. The table including the planet carrier
2. The planetary ring gear
3. The input shaft housing with the bevel pinion
4. The intermediate shaft with the bevel gearwheel

The table including the planet carrier was pulled off simultaneously. Four radial lifting point on table were used. The planet carrier is hanging on the table by a connection shaft (pos. 32). The circular nut is securing the vertical position of the carrier. This position was recorded before loosening the circular nut and separating the planet carrier from the table.

The condition of the planet bearing was worse than had foreseen. The outer race of one double spherical planet wheel bearing was cracked in several pieces, as shown on the picture below.

From two planet wheels the bores were heavily worn due to a slipping bearing outer race. The bore diameter was increased by 0.30 mm compared with original.

The double spherical roller bearings of these planet wheels require an approximate 0.05 mm shrink fit on the outer race of the bearing. In order to maintain this fit it was decided to oversize the bore of the two planet wheels and insert (shrink in) sleeves.
A drawing as well as a work procedure for manufacturing of the sleeves was provided by GBS International. The manufacturing of these sleeves was done by a gear company in Lahore. All critical diameters were ground and the material used was 4140 steel.

Planet wheel as received back from Lahore. Dimensions and surface finish was good.

Another less critical problem was found concerning the fit between the planet pins and the planet carrier. Two planet pins were found slightly loose. Long term problems are expected. The four rub screws per planet pin will prevent the pin from slipping or dropping out of the planet carrier.

A long term solution for this problem is provided in the recommendations.

The sharp edges around the damaged gear teeth of the sun pinion and ring gear were smoothened by hand grinding and polishing. This to avoid progression of the damage. Fortunately the damage on the sun pinion was limited to what has been seen during the earlier videoscopic inspection. However, on the ring gear one location was found with significant damage, see right picture.

The table of the gearbox itself was not disassembled due to the unavailability of spare parts (are ordered but not delivered). Also there was no indication of any problem. White metal particles were not found.

The lantern ring (pos. 29) was found somewhat scratched. Measuring tools to measure the inside diameter were not available due to its large size. This ring is also ordered but not yet delivered at the time of this repair. A proper clearance between this ring and the coupling case (pos. 25) is very important. Excessive clearance would create an insufficient flow of oil to the planet wheel bearings and planetary gears.

The ring gear was lifted from the lower gearbox housing and placed next to the gearbox. All teeth were polished and the damage on one tooth as described earlier was smoothened by hand grinding.

The input shaft housing was removed from the lower gearbox housing. No damage was found on the internals. The contact pattern and backlash was recorded just before removal and checked again afterwards.
Backlash before repair : 0.65 mm
Backlash after repair : 0.57 mm

The bearing clearance of the input shaft axial bearing was adjusted by machining of the bearing cover plate. This bearing cover plate also holds the oil seal.

Axial bearing clearance before the repair : 0.85 mm
Axial bearing clearance after the repair : 0.65 mm

The springs, used to push the inboard outer race of the axial bearing against the rolling elements, were still found acceptable.

The complete shaft had to be disassembled in order to replace the upper bearing. A special fixture was made to pull the bevel gear hub from the shaft. A 200 tons hydraulic jack was used in combination with 5 acetylene burners.

The complete axial weight of the intermediate shaft including the resulting gear forces of the bevel gear are taken by the upper bearing of the intermediate shaft. This bearing consists of two outer races with a spacer ring in between. The height of the spacer ring is accurately set by the bearing manufacturer to control the bearing inner clearances. The total height of the two outer rings and spacer was measured and found identical with the new bearing (176.08 mm). After installation of the new bearing the axial clearance of the bearing was recorded.

Axial bearing clearance before the repair : 0.40 mm
Axial bearing clearance after the repair : 0.37 mm

During the removal of the bevel gear from the hub it was noticed that the connection bolts were loose. Corrosion between the hub and the bevel gear wheel has been a result of this somewhat loose connection. During the assembly the bolts were tightened with the help of a hydraulic torque wrench. Torque levels were chosen according to DIN specification (2400 Nm).

The hub of the bevel gear was slowly heated during installation to overcome the interference fit between the hub and the shaft. Maximum temperature was closely monitored and did not exceed 110 degrees Celsius. During cooling of the hub on the shaft a press was used to avoid possible runout problems of the hub. The runout of the bevel gear was checked after re-installment of the shaft and was acceptable (<0.10 mm).
After re-installation of the intermediate shaft in the lower gearbox housing the backlash was recorded and found excessive (1.1 mm). The backlash was reduced by moving the bevel gear upwards. The liner (pos.51) was removed. A shim of 0.70 mm was made and installed in addition to the liner. The backlash was recorded again and found acceptable (0.57 mm).