The maximum pressure that a bearing can withstand is mainly a function of the bearing material. The bronzes that are common in sugar mills have a recommended maximum bearing pressures of up to 100 MPa for phosphor bronze and 50 MPa for tin-bronzes. Standard sugar mill practise limits the bearing pressure to about 10 MPa.
Materials For Plain Bearings
The two essential elements in a plain bearing are the bearing or bearing material itself, and the shaft or moving member. The bearing or bearing material is located in a housing or structure, and may or may not be integral with it. Separating these two elements is the lubricant, introduced, generally in the case of sugar mills, by external pressure feeding.
The material of the shaft or journal is established from considerations of strength and rigidity, and will invariably be steel.
Because the conditions under which bearings must operate in service may vary over a wide range, it is necessary that bearing materials be used which have certain desirable properties. Amongst these we must include such factors as
- mechanical strength;
- softness and low melting point;
- low modulus of elasticity;
- corrosion resistance;
- high thermal conductivity; and of course,
- economic considerations.
The most common bearing materials consist of
- white metals,
- copperbase alloys, and
- aluminium-base alloys.
White metals is a term used to include the tin and lead-base metals, broadly referred to as Babbitts (after Isaac Babbitt, 1839), and since such metals are highly competitive, they are recommended for most applications where the loading is not severe. Babbitt bearings are manufactured with the white metal lined onto steel, cast iron and copper base alloys. Since white metal suffers a reduction in fatigue strength with increase in temperature, and this reduction is a function of thickness, it is usual to limit the thickness to between about 0.100-0.175 mm, and thicknesses of only 0.025-0.050 mm are used with copperlead over the back-up material. White metal is not commonly used as a sgar mill bearing material
Copper-base alloys including lead-bronze, gun-metal and phosphor-bronze are widely used as bearing materials.
Lead-bronze is the cheapest, and is used for general service bearings. It has a low tendency to seizure, in common with the white metal bearings, and has greater fatigue strength to withstand higher temperatures. Lead bronze bushes are frequently used in the form of single, solid units, i.e. as bushes without the supporting shell surrounding the bearing material, as is required of the Babbitt or white metal bearing materials.
Gun-metal provides a relatively cheap and easy to machine material, having good bearing properties and capable of withstanding somewhat higher loads than the lead-bronze alloys. This alloy also has good resistance to corrosion in sea water.
Phosphor-bronze is used for heavily loaded bearings, where high frictional stresses are likely to occur. Because of the high hardness of this material, it demands the use of a hardened steel journal.
Typical Sugar Mill Bearings
Rein in Cane Sugar Engineering states that typically sugar mill bearings are tin bronzes with the following composition
Sugar mill shafts do not turn sufficiently fast for a hydrodynamic film of lubricant to be formed between the journal and the bearing. Consequently hydrostatic lubrication is required. This is achieved by supplying lubricant to the bearing under pressure. Under these conditions, attention must be given to the adequate supply of lubricant at all times, and in particular to the location of oil supply holes and grooves.
Bitumin based lubricants are often used in sugar mill bearings.
Bearing Loads and Sizes
Specific roll loads are in the range of 2 to 3 MN per square metre of projected roll area. This together with the allowable bearing pressure mentioned above indicates that the total bearing area should be about 20% to 30% of the projected roll area
It is usual practise to allow the top roll of a sugar mill to float in the vertical direction to:
- keep a nearly constant pressure on the mat of bagasse in the mill
- allow some throughput variation without sacrificing extraction
- protect the mill from damage from tramp iron
The correct precharge pressure which ensures that the top roll floats about its design position is important to ensure good extraction and to protect the mill from damage