25.13.
Gearbox Oil-leakage Prevention
A gearbox assembly uses many components and must be externally leak proof. From the perspective of sealing, the components which are joined together by machined pressure-face joints use static seals and the components which have rotary motion use dynamic seals.
25.13.1.
Static Seals
A static seal is positioned between two pressure faces so that when they are clamped together, the seal deforms and conforms to their surface roughness, waviness, and misalignment
Fig. 25.28. Splash-feed lubrication system for gearbox.
Fig. 25.29. Forced-feed lubrication system for gearbox.
within a limit. These seals are used between the main gearbox casing and the front, top, and back covers, or in some designs between split half-casings. The common static seals are non-metallic gaskets such as paper, synthetic-rubber O-rings, and mixed compounds of both the setting and non-setting types. Often compounds are applied on gaskets to provide additional support against leakage. With aluminium-alloy casings, particularly if the casing is of the split half type, the two halves are bolted directly together with only sealing compound as an in-between packing.
25.13.2.
Dynamic Seals
To prevent oil leakage between a rotating shaft and its bearing housing, either a spiral thread clearance seal or a contact radial-lip seal is used.
Spiral-thread Clearance Seal.
This form of seal contains a parallel portion of a revolving shaft which fits inside an extended sleeve housing, and an internal spiral (or more accurately, helical) groove is machined inside the sleeve bore (Fig. 25.30). A small clearance is provided between the shaft and the sleeve.
The primary-shaft bearing is lubricated by splashing of oil against the side of the ball bearing. Some of this oil creeps between the inner and outer bearing race tracks and drips into the outer bearing housing and on to the primary shaft. When sufficient oil is trapped between the shaft and the housing-sleeve bore, the revolving shaft develops a pumping action so that oil is further moved along the shaft. When the oil encounters the helix or spiral edge, it is pushed into the spiral rectangular groove. Consequently the existing oil covers the revolving shaft and then drags the oil along the spiral-groove path, back towards the bearing and the drain-hole. Each groove collects a definite quantity of oil, but excess-oil overspill decreases along the shaft.
In some designs, instead of having the spiral groove inside the sleeve, it is machined on the outside of the shaft, and the inside sleeve bore has a smooth surface finish.
Dynamic Contact Radial-lip Seal.
This seal is made up of three parts ; these include (i) a short steel cylindrical sleeve flanged at one end, (ii) a flexible synthetic rubber lip seal, which is bonded to the steel flanged sleeve, and (Hi) a circumferential garter or band spring, which is located at the rear end of the rubber lip (Fig. 25.31).
The steel sleeve which is force fitted in its housing bore, locates and supports the flexible seal. The rubber seal contains a flexible web which is always concentric with the shaft, even when the shaft is deflected slightly to one side, under load. The large included angle between the face angle and the clearance angle «E> and 0 respectively) stiffens the sealing point against local distortion. To improve the contact between the shaft and the knife-edge rubber lip, a circumferential spring bears on the back of the seal knife-edge rubber. The knife edge rubbing contact with any oil film formed on the shaft surface tends to create a viscous oil joint which rejects any further outward leakage.
