Shaft, Shaft Sleeves and Stuffing Boxes for Pumps
Shaft and Shaft Sleeves
Pump shafts transmit power from the prime mover to all rotating parts of the pump, especially the impeller. The torque acting on the shaft varies with situations. During starting period, shaft experiences heavy load compared to that during running conditions. Most of the pumps uses a short length shaft sleeve around pump shaft. The function of the shaft sleeve is to protect shaft from wear and tear. This arrangement is common in pumps using gland packing sealing system, where only shaft sleeve need to be renewed after sleeve getting heavily eroded.
Loads involved in Shafts
- Weight of the parts, and
- Both the radial and axial hydraulic forces.
If a pump is to handle hot liquids, the shaft is designed to withstand thermal stresses and if it is to handle corrosive liquids, the materials used should be corrosion resistant.
Pump shafts have to withstand erosion, corrosion and wear at stuffing boxes, leakage joints, internal bearings and in waterways, etc. To protect the shaft, and also to give it special properties, eg. higher resistant to wear, renewable sleeves are fitted on the shaft.
Stuffing Boxs have the primary function of protecting the pump against leakage at the point where the shaft passes out through the pump casing.
Different arrangements can be provided and the design depends upon the operating conditions (pressure or vacuum) and type of fluid being pumped (water, oil, toxic liquids, etc.) Improved sealing efficiency and life of seals is achieved by means of providing special arrangements for cooling or lubricating the packing and the shaft in way of the stuffing box.
In the ordinary stuffing box, the sealing between the moving shaft or shaft sleeve and the stationary portion of the box is accomplished by means of rings of a semi-elastic packing material which is compressed in place with the help of the stuffing box gland. Selection of packing material depends upon the type of fluid being pumped and the reliability required. Packing materials wear the shaft in way of the stuffing box and care must be taken while selecting the packing.
Functions of Sealing
- To prevent air leaking into the pump.
To prevent liquid leaking out of the pump.
The actual sealing surface consists of the axial rotating surfaces of the shaft or the shaft sleeve and the stationary packing. In extreme cases, frictional heat causes damage to the packing and the shaft, and therefore, a proper sealing cannot be effected. If the sealing of the stuffing box is desired, a "lantern ring" or a "seal cage" is used. This separates the rings of packing into sections. The lantern rings or the seal cages are usually axially split for the ease of assembling. Refer to the figure above.