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In determining the proper weight on the bit the following four major factors are to be consider by the driller while drilling a hole.

The weight which is carried within the drill string is governed by the size and weight of the drill pipe, drill collar, type of bit and type of formation being cut.  If the load capacity of string exceeds the limit, it will cause undue strain and fatigue resulting premature failures.  From the technical efficiency point of view correct size of drill pipes should be used, but, still it does not meet the full requirement of economic drilling operations.The weight which is carried by drill string while maintaining the hole straight is being controlled by the type of bit, speed of rotation, type and nature of formation, type and size of stabilizer and drill collars used.  The weight applied on the bit will cut the bottom of the hole horizontal and will keep the hole straight. Horizontal formations of any character are less likely to cause deviations from the vertical. The steeper the dip the greater the tendency for the bit to leave the vertical.  Most important and prime factor in vertical drilling is weight concentration immediately above the bit.  This weight must be sufficient as required by the bit at bottom and also to hold the drill string in tension below the rotary table.  If, the bit is attached to the drill pipe without the use of drill collar shows that drilling a vertical hole very little weight is placed on the bit.  The minimum bit collar weight is governed by the weight required by the bit to cut the formation and balance weight to keep drill string the tension below the rotary table.

There are practical limitations to the length of the drill collars.  The ideal drill collar must have a maximum diameter and weight just above the bit and then decreasing in a proportion to the size of the drill pipe and tool joint.  Generally, in water well drilling 10' to 20' length drill collars are the most adoptable.  From the field experience in various flat lying formation it is gathered that 50-75% drill collar weight is mostly concentrated on the bit in normal drilling.  As the inclination of beds increases the drill collar weight decreases correspondingly until the formation approached verticality.  During this period the bit at the bottom do not cut the formation with maximum efficiency, because most of the weight of the drill collar is transmitted to the walls of the bore hole drilled.

Thirdly, a limiting factor of the weight carried by the bit during the course of drilling is the weight carrying capacity of the bit being used.  The weight required to penetrate a formation varies directly proportional to the diameter of the bit.  The load carrying capacity of the bit varies with the bearing sizes.  The size of bearings varies at greater rate than the diameter of bit but at a lesser rate than the area of the hole.  Thus a 16'' rock roller bit can carry a load 3-4 times as heavy as can a 8'' rock roller bit or about 3-4 times per inch of hole diameter.

Fourthly and lastly, the amount of weight at which the hole is drilled most rapidly.  The impact of roller cutting tooth may cut the formation even, if, the weight carried is less than the required weight, but the rate of drilling will be slow then the efficient rate.

This proper balance can not be defined in exact figures because of many unknown factors like the lack of uniformity in the materials being cut, wear on the bit, etc., but reasonable limits can always be found. The proper balance of these four consideration results in maximum drilling efficiency under no circumstances the weight carried on the bit exceeds the weight of drill collar, irrespective of the tools attached with the drill string.  The weight should not exceed more than 75% weight of the drill collar. In normal drilling conditions 1000-3000 lbs of weight for each inch of diameter of the bit is the most ideal (Table-1).

In addition to the proper weight applied on the bit, it is equally important that this weight is maintained uniformly, otherwise, during the period of excessive weight it may cause drill pipe fatigue, deviation of hole etc. etc.  During this period the rate of drilling reduces considerably.  Therefore, even if, calculated weight is placed on the bit which cuts most efficiently at faster rates, but during the period of excessive weight the rate of drilling will be reduced.

Generally, the only way to determine the proper weights to be carried on the bits under given conditions is by the experience.  The driller should start with light weight and increase as required, rather than to begin with heavy weight and then decreasing.  The weight carried should always within the concentration at the bottom of drill string.