Sunday, 12 November 2017

TCB Cone Geometry Design

The cones are mounted on bearings which run on pins that are an integral part of the bit body.

Cone Offset:
The drilling action of a rolling cutter bit depends to some extent on the offset of the cones. As
shown, the offset of the bit is a measure of how much the cones are moved so that their axes do not intersect at a common point in the center of the hole.

roller cone bit and offset angleOffsetting causes the cones to slip as they rotate and scrape the hole bottom much like a drag bit. This action tends to increase penetration rate in soft formations.

In hard formations where the rock must be fractured or broken, scraping contributes little to rock removal. In addition scraping against a hard formation is very abrasive and can wear the teeth down quickly. For these reasons hard formation bits are designed with little or no cone offset.

Cone offset angle is expressed as the angle the cone axis would have to be rotated, to make it pass through the center line of the hole.

Cone offset angles vary from 4 degrees for bits used in soft formations to zero for bits used for extremely hard formations.

Maximum offset
(3° Skew Angle)
Increases gouging, scraping action
Limited offset
(2° Skew Angle)
Limited cutter action
No offset
(0° Skew Angle)
Minimized gouging, scraping action

All three cones have the same shape except that the No. 1 cone has a spear point. One of the basic factors to be decided, in the design of the cones, is the journal or pin angle.

The journal angle is formed between the axis of the journal and the horizontal. Since all three cones fit together, the journal angle specifies the outside contour of the bit.

journal angle in roller cone bit

Low angle allows a prominent cutter action and permits greater tooth depth.
34° to 36°
Decreases cutter action
Minimizes cutter action

The use of an Oversize Angle increases the diameter of the cone and is most suitable for soft formation bits. Although this increases cone size, the gauge tip must be brought inwards to ensure the bit drills a gauge hole.

roller cone bit design cone and journal angle and oversize angle

One important factor which affects journal angle is the degree of meshing or interfit (i.e. the distance that the crests of the teeth of one cone extend into the grooves of the other).

The amount of interfit affects several aspects of bit design:
  • It allows increased space for tooth depth, more space for bearings and greater cone thickness
  • It allows mechanical cleaning of the grooves, thus helping to prevent bit balling
  • It provides space for one cone to extend across the centre of the hole to prevent coring effects
  • It helps the cutting action of the cones by increasing cone slippage.

interfit roller cone bit

In some formations, it is advantageous to design the cones and their configuration so that they do not rotate evenly but that they slip during rotation. This Cone slippage, as it is called, allows a rock bit to drill using a scraping action, as well as the normal grinding or crushing action.

Cone slippage

can be designed into the bit in two ways. Since cones have two profiles: the inner and the outer cone profile, a cone removed from the bit and placed on a horizontal surface can take up two positions. It may either roll about the heel cone or the nose cone. When the cone is mounted on a journal it is forced to rotate around the centre of the bit. This “unnatural” turning motion forces the inner cone to scrape and the outer cone to gouge. Gouging and scraping help to break up the rock in a soft formation but are not so effective in harder formations, where teeth wear is excessive.

Cone slippage can also be attained by offsetting the axes of the cones. This is often used in soft formation bits. To achieve an offset the journals must be angled slightly away from the center. Hard formation bits have little or no offset to minimise slippage and rely on grinding and crushing action alone.

roller cone bit and cone slippage

Basic cone geometry is directly affected by increases or decreases in either journal or offset angles, and a change in one of the two requires a compensating change in the other. Skidding-gouging improves penetration in soft and medium formations at the expense of increased insert or tooth wear. In abrasive formations, offset can reduce cutting structure service life to an impractical level. Bit designers thus limit the use of offset so that results just meet requirements for formation penetration.

offset and lournal angle in design of a roller cone bit

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