engineering – seals and bearings?
describe the principles of operation of the following engineering components, giving a typical application for each:
static seal or dynamic seal
packed gland
rolling element bearing
plain or journal bearing
1.Static seal or dynamic seal and packed gland are the parts of a centrifugal pumps for gland sealing methods.
Dynamic sealing — or hydrodynamic sealing — is accomplished by a combination of pumping vanes on the rear shroud of the impeller and a secondary expeller located behind the impeller, which rotating with the shaft, impart a rotary movement on the liquid in the pump resulting in the liquid centrifugally moving away from the center shaft penetration in the case.
An equilibrium in the back of the pump is achieved between the pressure of the liquid in the pump and the atmosphere, creating an air void at the shaft penetration of the case while the pump is running.
The dynamic seal provides for zero leakage of the pumped product without the need for costly mechanical seals, packing, and lip seals or in most cases seal water and the associated supporting seal water systems. Product dilution due to seal water is further eliminated.
With no rubbing seal components to fail over time, the dynamic seal can operate reliably over the life of the hydraulic wet end of the pump.
Dynamic seals can offer reliable service in difficult applications that may include; varying amounts of solids, liquids that are close to saturation prone to crystallizing, scaling liquids and where operational upsets such as cavitation, dead heading of the pump and even dry running can occur.
2.A rolling-element bearing is a bearing which carries a load by placing round elements between the two pieces. The relative motion of the pieces causes the round elements to roll with very little rolling resistance and with little sliding.
One of the earliest and best-known rolling-element bearings are sets of logs laid on the ground with a large stone block on top. As the stone is pulled, the logs roll along the ground with little sliding friction. As each log comes out the back, it is moved to the front where the block then rolls on to it. You can imitate such a bearing by placing several pens or pencils on a table and placing your hand on top of them.
3. The packed gland is used primarily for sealing process valve shafts (axial movement) and for process pump shafts(rotary movement). The packed gland has provided a low tech. convenient solution throughout the history of engineering. In modern times the packed gland is being replaced by more exotic solutions.
The packed gland seal for pump applications is now rarely fitted to new pumps. This is due to it’s high maintenance requirements. When fitted it is mostly used in conjunction with long coupled bed plate mounted pumps. Specific operating conditions require distinctly different types of packed gland seals. They require regular checks and maintenance adjustments.. Proper lubrication of the gland packing requires a certain leakage rate. Special manufacturers recommendation’s are to be observed individually. Service life expectancy is between 1 and 2 years, this can sometimes extend to several years on favourable conditions. Extremely bad fluid conditions (sediments, additives, overheating) can however drastically cut short their service life. Packed glands should preferably be used in conjunction with shaft sleeves in order to avoid damage to the shaft by aggressive fluids or due to improper treatment of the packed gland.
4.Journal Bearings :
Industrial machinery with high horsepower and high loads, such as steam turbines, centrifugal compressors, pumps and motors, utilize journal bearings as rotor supports.
One of the basic purposes of a bearing is to provide a frictionless environment to support and guide a rotating shaft. Properly installed and maintained, journal bearings have essentially infinite life.
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