Required Weight on Bit

From time to time, various customers inquire about the correct amount of Weight on Bit to optimize the drilling performance. This is of particular importance when drilling deep holes. Keep in mind, “Weight on Bit” means specifically the weight of all the Drill Rods, the Hammer minus the weight of the Piston, and the Top Drive Rotary Head.

For shallow holes the hydraulic down pressure applied to the Rotary Head through the pull down cylinders provides the necessary additional force. This additional pull down force is usually shown as a PSI value on the Rig gauges. To convert this figure to a weight value you will need to multiply the PSI reading by the area of the pull down cylinders. This can then be added to the weight of the Rods, Rotary Head, and Hammer minus Piston, to give the total “Weight on Bit”.

As the hole deepens with the addition of extra rods, a point is reached where the pull down force needs to be reversed to create a “hold back force” thus preventing too much weight being applied to the Bit. Since there are so many different types and sizes of rigs, it is not possible to give specific guidelines on the amount of additional down pressure as shown by the gauge. This becomes part of the necessary tuning by the drill operator. Regarding the actual figures for required weight the easiest way to calculate this is to use the two simplified formulae below:

1) When working with air pressure below 250PSI, multiply the Bit Head Diameter (in inches) by 500.

Example :- A Button Bit with a 6” diameter Head would require 3000Lbs “weight on bit”.

2) When working with air pressure at 250PSI and above, multiply the Bit Head Diameter (in Inches) by 800

Example:- A Button Bit with a 6” diameter Head would require 4800Lbs “weight on bit”. Because these formulae have been simplified to allow for quick calculation you may find that, dependant upon ground conditions, slight alterations may need to be made to these values. Again this a necessary part of the tuning process performed by the drill operator.


Required Weight on Bit From time to time, various customers inquire about the correct amount of Weight on Bit to optimize the drilling performance. This is of particular importance when drilling deep holes. Keep in mind, “Weight on Bit” means specifically the weight of all the Drill Rods, the Hammer minus the weight of the Piston, and the Top Drive Rotary Head. For shallow holes the hydraulic down pressure applied to the Rotary Head through the pull down cylinders provides the necessary additional force. This additional pull down force is usually shown as a PSI value on the Rig gauges. To convert this figure to a weight value you will need to multiply the PSI reading by the area of the pull down cylinders. This can then be added to the weight of the Rods, Rotary Head, and Hammer minus Piston, to give the total “Weight on Bit”. As the hole deepens with the addition of extra rods, a point is reached where the pull down force needs to be reversed to create a “hold back force” thus preventing too much weight being applied to the Bit. Since there are so many different types and sizes of rigs, it is not possible to give specific guidelines on the amount of additional down pressure as shown by the gauge. This becomes part of the necessary tuning by the drill operator. Regarding the actual figures for required weight the easiest way to calculate this is to use the two simplified formulae below: 1) When working with air pressure below 250PSI, multiply the Bit Head Diameter (in inches) by 500. Example :- A Button Bit with a 6” diameter Head would require 3000Lbs “weight on bit”. 2) When working with air pressure at 250PSI and above, multiply the Bit Head Diameter (in Inches) by 800 Example:- A Button Bit with a 6” diameter Head would require 4800Lbs “weight on bit”. Because these formulae have been simplified to allow for quick calculation you may find that, dependant upon ground conditions, slight alterations may need to be made to these values. Again this a necessary part of the tuning process performed by the drill operator.
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