Rubbing wear

Osmium melts at 3045°C. It is a hard, heavy element that is easily oxidized in air to toxic OSO4. The ease with which this oxide is formed demands caution. Osmium is difficult to work with because of its hardness at low temperatures and the possible formation of OSO4 at elevated temperatures. It is very resistant to rubbing wear, resistant to H2SO4 and 40% HF at ambient temperatures, and 36% HCl at 100°C but is easily attacked by oxygen, HNO3, and aqua regia. 

Table 6 shows typical adhesive dry rubbing wear values for surface treatments and coatings. These were determined from a pin-on-plate... 

When two metallic surfaces are lubricated in a hydrodynamic regime, the oil film is stable and problems of wear are not very important. In severe service, the film can be destroyed from then on the metallic parts rubbing on each other can cause first metal loss and then even the seizing of the parts by welding. 

In the second type of mill (Fig. 20-28), a cyhndrical grating is provided beneath the rotor for product discharge. Some hammer crushers are symmetrically designed so that the direction of rotation can be reversed to distribute wear evenly on the hammer and breaker plates. When such a Pennsylvania nonreversible hammermill is used for reduction, material is broken first by impacl against hammers and then by rubbing action (attrition) against screen bars. Performance data of Pennsylvania reversible hammer mills are shown in Table 20-14. 

The causes of vibration have already been discussed. An increase in vibration over a period of time is generally caused by loss of ahgn-ment, setthng of the foundation, or sticking of some expansion feature such as a pipe or a pedestal. Other causes are wear in the teeth of a flexible couphng, an internal rub in the unit, loss of bearing oil, and bearing wear. (Startup vibration is discussed later under temperature bow.)... 

Wear and increased leakage from the glands are common. Carbon rings may need replacement after I or 2 years of operation. A unit with labyrinth packing may never need packing replacement. This depends upon operation. If a unit is started quickly with a temperature bow in the shaft, the result is a rub in the labyrinth, and then all packing may need replacement. 

A bent shaft is physically bent and distorted. Placing the shaft into a lathe or dynamic balancer and rotating it will reveal the distortion. If a bent shaft is installed into a pump and run, it will fail prematurely, leaving evidence and specific signs on the eircumferenee of close tolerance stationary parts around the pump s volute circle. The shaft will exhibit a wear spot on its surface where the elo.se tolerance parts were rubbing. 

Wear and rubbing between close toleranee rotary and stationary elements in the pump leading to their failure. 

The slurry seal with the evacuation line is designed to handle/evacuate suspended solids, crystals, sediment, and dirt in the pumpage. The seal s springs are located out of the fluid. The o-rings move and rub across a clean surface as the faces wear. The o-rings are placed away from the heat generated by the faces. 

This dial indicator is fixed to the volute mounting adapter collar of the pump and the needle is on the shaft (Figure 14-21). The shaft should be moved radially by hand (see the arrows) up and down. Note the movement in the indicator. This is a check of the radial tolerance in the bearing. Some people use the word run out . Radial dellection causes misalignment of the rotating and stationary faces of the mechanical seal. This shortens the seal life by eausing drive pins and springs to wear and rub in relative motion. 

Several steps can be taken to maximize the run time for the reciprocating compressor. Since wear is a function of rubbing speed, the piston speed can be kept to a minimum. Chapter 3 made recommendations for piston speed. Reliability problems due to valves are reputed to account tor 40% of the maintenance cost of the compressor. Valves are the single largest cause for unplanned shutdowns. Basically, valve life can he increased by keeping the speed of the compressor as low as practical. At 360 rpm, the valves are operated six times a second. At 1,200 rpm, ihc valves operate 20 times a second or 1,728,000 times in a day. It is not difficult to understand why the valves are considered critical. To keep the reliability in mind, valve type, material selection and application considerations such as volume ratio, gas corrosiveness, and gas cleanliness need attention by the experts. One final note is that while lubrication is an asset to the rubbing parts, it is not necessarily good for valve reliability. 

Attrition The rubbing of one particle against another in a resin bed frictional wear that will affect the site of resin particles. 

Abnisbohne,/. jequirity bean, abrussecht, a. fast to rubbing, not crocking, abrussen, v.i. (of colored goods) rub off, crock, abrtisten. v.i. disarm. —v.t. dismount, abrutschen, v.i. slip, slide (off or down). — v.t. wear off or out. 

Abschl mmung, /, washing, decantation, abscbleifen, v.t. grind off, grind, rub off, abrade polish sharpen.—v.r. wear, wear off, wear smooth. 

Keep the packing rings in the order they are extracted from the gland box. This is important in evaluating wear characteristics. Look for rub marks and any other unusual markings that would identify operational problems. 

Abrasion or corrosion due to Race misalignment Initial coarse finish on shaft Tight rubbing seals Seal rubbing Seal wear... 

Seal rubbing or Cage wear Cocked races Flatted roller or ball Seal failure... 

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