Gear Lapping: Achieving High Flatness and Surface Finish in Aerospace Fuel and Hydraulic Systems
In aerospace fuel and hydraulic systems, gear faces with integral shafts require high flatness and surface finish to ensure efficient operation and prevent leaks. However, due to the presence of shafts, it becomes challenging to present the faces needing lapping to a conventional lapping plate. In such cases, gear lapping using specialised automated systems can provide a solution. Kemet, a leading provider of precision lapping and polishing systems, has developed automated systems to overcome this challenge. These systems replace hand finishing techniques that are time-consuming and variable. With Kemet's gear lapping service, parts with integral shafts can be processed in their sub-contract facility, generating flatness of less than 0.0005mm (0.5 microns). To avail of this service, all that is required is to send a drawing of the part needing processing along with the desired quantity. Kemet will calculate exactly what is needed and issue a quotation that includes a bespoke annular groove plate and an optical flat specifically designed for the part.
Gear lapping is a process that involves the use of abrasive compounds and specialised equipment to remove small amounts of material from the gear faces, resulting in a high flatness and surface finish. The process is typically carried out on gears that are already hardened, to avoid distortion during the lapping process. During the gear lapping process, the gear is placed on a lapping plate that is covered with an abrasive compound. The plate rotates at a slow speed while a pressure pad is used to apply a small amount of pressure to the gear face. The gear is rotated at the same time, creating a sliding motion between the gear face and the lapping plate. The abrasive compound acts as a cutting tool, removing material from the gear face and producing a smooth and flat surface. However, in the case of gears with integral shafts, the gear cannot be placed directly on the lapping plate. To overcome this, Kemet has developed an automated system that uses a specialised annular groove plate. This plate has a groove around its perimeter that allows the gear to be placed with its integral shaft in the centre of the plate. The groove also allows for the abrasive compound to be contained within the area of the gear face that needs lapping.
To ensure high flatness and surface finish, Kemet also uses an optical flat during the gear lapping process. The optical flat is a precision instrument that is placed on top of the gear face during lapping. The flatness of the gear face is measured by the interference patterns created between the flat and the gear face. This allows Kemet to monitor the flatness of the gear face during the lapping process and ensure that it meets the required specifications.
Test Requirements: To Lap to drawing
Component/Material: Batches of 50 bronze gears
Machine Type: Kemet 36 lapping machine
For the first stage you need to place 8 Gears in the Conditioning ring and lap them with no additional pressure on the cast iron lapping plate for 5 minutes on each side. Doing this takes off the high spots so helps keep the part within the parallelism spec. Next you need to clean of and measure all of the parts so you can calculate the correct process time for the final sizing stage.
Once you have worked out the time required for this secondary stage, you will need to place 5 Gears in a fixture and lap them under a 20KG weight (Standard 36” machine weight) for the calculated time. For example, 2 minutes and 30 seconds on each side. After this is done you need to remove the parts from the nest then clean them in an ultrasonic cleaner and rinse them in deionised water to remove all dirt and stains. After the parts are clean and dry you must measure all parts to make sure they are all in the required size and parallelism.
| Gear Lapping Process | |||
|---|---|---|---|
| Stage | Plate/cloth type | Abrasive type/grade | Process time |
| 1 - Removal of high spots | Cast Iron | Kemox 0-800 s 14µ | 5 minutes |
| 2 - Sizing | Cast Iron | Kemox 0-800 s 14µ | 2 minutes 30 seconds |
