Construction of bearings for worm gear reducer
A separate insert or bushing is usually provided to form the bearing surface for supporting the shaft of worm gear reducer in a sleeve bearing.It is made of a material that is know to have desirable qualities of high load capacity and low friction for worm gear reducer.The types and details of construction for sleeve bearings can vary over a considerable range.
For a solid bronze bushing,a type of bushing that is in wide use for worm gear reducer.It can be cast,machined from bar stock, or formed from tubing or sheets for worm gear reducer.It is usually held in place by a light press fit.The bearings of worm gear reducer shown in which consist of a steel or bronze back with a thin lining of babbitt or one of the numerous other bearing alloys.Such bearings of worm gear reducer can be made either all around or split on the centerline of the shaft.A wide variety of types and sizes of such bearings is available from stock ready to use.They may be obtained solid,split longitudinally,or flanged,and with many kinds of oil grooves.
The oil ring bearing of worm gear reducer which has a cast-iron back with a babbitt liner cast in place to obtain a better oil supply in slow-speed service,the ring is sometimes replaced by an endless loop of chain hanging on the shaft.
Clearance and oil Grooves of worm gear reducer
In addition to choosing the material,the designer must specify the shaft clearance and the oil grooving for the bearing of worm gear reducer.The clearance depends,to some extent,on the desired quality.Small clearances can be maintained with high-grade workmanship for worm gear reducer ,but when costs must be reduced,the clearances usually are made larger.
For small clearances,the surface roughness of the shaft of worm gear reducer and bearing must be considered.These surfaces,when examined with a microscope,are usually far from being geometrically smooth.In many cases,the roughness consists of two orders of irregularities.Over most of the surface of worm gear reducer there are relatively small,low irregularities at close intervals.In addition,at relatively large intervals,there are high peaks and deep valleys.Roughness is usually measured from the tops of the high peaks.To accentuate the roughness,it is customary to prepare samples with a vertical scale that is 25 times as great as horizontal scale.Profilometers give readings for either the root mean square or the arithmetic average of the roughness.The predominant peak roughness can then be had by multiplying by a suitable factor.For example,for ground surfaces,the arithmetic average roughness should be multiplied by 5 to obtain the predominant peak roughness.
It shows the surfaces of worm gear reducer for journal and bearing located so that the high peaks are just in contact.It has been proposed that the condition illustrated be taken as the limit at which hydrodynamic lubrication can be assumed to exist.Surface finish should,thus,be specified and closely controlled if the design calculations indicate that the bearing will operate with a very thin film.Experience has indicated,however,that other factors,such as misalignment,deformation,foreign matter in the oil,and fatigue due to dynamic loads,are the cause of most bearing failures from worm gear reducer.
Oil grooves with carefully rounded edges should be provided to distribute the oil over the entire surface of worm gear reducer.It is very important,however,that no grooves be placed in the load-carrying portion of the lining.When so placed,such grooves merely provide an easy exit for the oil and prevent the formation of the film and the high pressures necessary to support the load.A good rule of worm gear reducer is to use the least possible amount of grooving.Sometimes, the chamfer between the bearing and cap provides channel for the lateral flow of the lubricant.It shows the reduction in load capacity due to an incorrectly located oil groove.If the load on the bearing with middle groove is not reduced,operation may take place with a dangerously thin oil film.If a belt pulley of worm gear reducer is adjacent to a bearing,care should be taken that the forces from the belt do not pull the shaft toward the side where oil grooves may be located and where a satisfactory oil film cannot be formed.
In a similar manner,a circumferential groove reduces the load-carrying qualities.An oil collar is very effective means of distributing the lubricant.However,for worm gear reducer,an oil collar practically cuts the bearing in two,and the sum of the loads carried by each part is less than that carried by a single bearing of the total width.Forced-feed-lubricated bearings frequently are constructed without oil grooves.
The most important point to be considered when choosing the method of lubricating a bearing of worm gear reducer is the reliability of the oil supply.Because of low initial cost,many bearings are lubricated by hand oilcans,drip or wick oilers,oil-soaked waste,or grease.As previouslymentioned,such bearings usually operate in the region of boundary lubrication,with its attendant high friction and danger of seizure,worm gear reducer should the scanty oil supply become temporarily deranged.
Safety lies in having a copious oil of worm gear reducer supply at all times.Such a supply can be provided by an oil ring and reservoir,or by piping the oil to the bearing under pressure from a central pumping station.Whatever method is used,close attention to the mechanical details is required if satisfactory operation is to be expected.
Means for preventing leakage of worm gear reducer and loss of oil at the ends of the bearing must often be provided.Felt gaskets or patented oil seals are in wide use.Sometimes the outer housing is enlarged to accommodate a small disk fastened to the shaft.The oil of worm gear reducer has leaked past the side of the bearing is thrown off the disk by centrifugal force and caught by the housing.Often a circumferential groove at the end of the bearing is provided with drain holes to return the oil to the reservoir.
Grease of worm gear reducer is widely used for exposed locations because it is easier to retain in the bearing.Investigations have shown that the action can be hydrodynamic with a pressure distribution somewhat like that for oil.
