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INSTALLATION OF SPECIAL THRUST UNITS INTO BEARING CHOCKS OF ROLLINC MILLS

1. Reasons for development

Axial forces often cause failure of the work chock bearings of continuous sheet mills. It is proved by the fact that 80 - 90% of failed bearings are located in the fixed chocks (on the roll change side).

The main limitation of numerous attempts to relieve axial load pressure on four-row bearings of work rolls by installation of conventional thrust units (provided, as a rule, with heavy double-row tapered roller radial-thrust bearings) was bulky size of these units and low reliability of the radial-thrust bearings when operated as purely thrust-resisting ones.

2. Main design and functioning features

The feature of the thrust unit, designed for the work roll chocks of the continuous hot rolling wide-strip mill 1700 and represented on the figure below, is, first of all, the use of the special double-action tapered-roller thrust bearing ( 189772 ХМ, С=1400,0 kN, С0=7450,0 kN).

We designed the bearings of this series (189000) especially to react axial loads in the roll supports of rolling mills. They highlight the feasibility of mounting into a thrust unit without adjustment of axial clearance, equalization of loads on rollers due to possible misalignments and skewing in the unit, as well as increased rotation rate as compared to known roller thrust bearings. The main characteristics of tapered-roller thrust bearings of this standard size series developed with the assistance of NKMZ (Novo-Kramatorsk Machine-Building Works) with reference to problems of modernization of rolling mills are listed in the table 1.

Fig.1. Thrust unit with the special thrust bearing
for fixed work chock of finishing stands of the hot strip mill 1700

Thrust unit 4 (Figure 1) with double-action tapered-roller thrust bearing 5 has minimum axial size and, at the same time, much higher durability as before and low heat release, there being no need for special cooling or forced-feed/circulatory lubrication.

Thrust unit 4 mentionned above is installed, as is shown in the figure 1, on the site of the front cap of fixed chock (on the roll change side) 2, so that the modification of the chock shell is not necessary. What is needed is only the modification of the working roll tail-end on the idle side, which has no impact on the working capacities of the shaft.

Table 1. DOUBLE THRUST TAPERED ROLLER BEARINGS SERIES 189700, 189800, 189300

Thus, axial forces from roll 1 onto chock 2 could be transferred only by one of the roller rows of the thrust bearing 5, while four-row bearing 3 ( 77779) can react only radial load, as its inner races are not axially fixed on the roll surface.

We have also designed some alternate constructions of the thrust units for chocks with four-row tapered roller bearings having tapered bores where the inner races of the four-row bearings stay fixed while the outer races are floating.

Realization of similar thrust units may be perspective in back-up chocks of sheet mills where their application allows to forgive a construction and reduce the price of equipment and at the same time to reduce axial dimensions of thrust units in comparison with traditional decisions with use of double row tapered roller bearings. Thus there is also an increase in existing limited axial dimensions of load ability of bearing units.

Example of such development providing use of the thrust bearing № 1898/510 in oil film bearing «ПЖТ 1180» of the sheet mill 2800 back-up chocks, offered togetner with EZTM (Electrostal, Russia) for increase (in existing dimensions, a load ability of the back-up chocks of sheet mill stand 2800 is presented on Figure 2.

Figure. 2.

3. Efficiency and application

Due to high axial load-carrying capacity of the thrust bearings, their estimated service life is about one year. Taking into account nature of failure of the four-row bearings of working chocks, the estimated reduction of the total consumption of the working roll bearings will be more than expected by a factor of 2,0 times.

The bearings of this series belonging to smaller standard-size (189740 Х and 189750 Х), designed by us and manufactured by the «ВНИПП» Bearing Factory (Moscow, Russia) in 1990 for the bar mill of the Byelorussia Metallurgical Works (Zhlobin, Byelorussia) have been successfully operated there since 1992.

Bearings 189757 ХМ, and 189772 ХМ, 189778 ХМ and 189892 ХМ designed specially for roll supports of both cold and hot rolling mills put in quantity production by Samara Bearing Factory (Samara, Russia), may be delivered if accordingly ordered.

Starting the production line and manufacture of a batch of thrust bearings type 189000 belonging to alternate standard size series (shown in the table) could be organized within a year. Design and delivery of the thrust units could be realized by us within 6 months; adjustment and putting into operation – within two months from the moment of delivery.