How to select a right kelly bar?

Selecting a kelly bar (a key component in rotary drilling rigs used for foundation drilling) requires careful consideration of several factors to ensure optimal performance and longevity. Here are the key points to consider when selecting a kelly bar:

- Kelly Bar Type

Interlocking Kelly Bar:

Best suited for hard or dense soils, such as rock, stiff clay, and gravel.

Ideal when high torque is needed to break through difficult ground conditions.

Recommended for applications where precision and stability are crucial.

Friction Kelly Bar:

Suitable for soft or medium soils, such as sand, loam, or loose gravel.

Works well when the ground offers less resistance, allowing smoother drilling.

Commonly used in shallow drilling applications where torque requirements are lower.

- Drilling Depth

The depth of the drilling project dictates the length of the kelly bar. Longer kelly bars are required for deeper boreholes. The drilling depth also determines the number of kelly bar sections.

The formula for roughly calculating the drilling depth of the Unidrill kelly bar is:

sections*length of each section-sections*1.5m

For example, the kelly bar has 3sections, length of each section is 14m, then the drilling depth is:

3*14m-3*1.5m=37.5m

This is a simplified example, and actual calculations should be based on actual project.

- Torque Requirements

The required torque depends on the soil conditions, drilling diameter, drilling depth and the drilling rig. Ensure the kelly bar can handle the rig's torque and the ground's resistance.

Example Calculation of Torque:

For a rough estimate, the required torque (T) in Nm can be approximated as:

T=K×D×H

Where:

K is a coefficient based on soil resistance (this can range from 0.5 to 3 depending on soil type).

D is the diameter of the hole (in meters).

H is the depth of drilling (in meters).

For example, if you are drilling a 1-meter diameter hole to a depth of 30 meters in medium-density soil (K = 2), the estimated torque would be:

T=2×1×30=60 Nm

This is a simplified example, and actual torque calculations should be more detailed and based on specific project conditions.

Beside, the torque of kelly bar shoule match the Kelly bar torque with the capacity of your drilling rig. Over-torqueing beyond the rig's capacity can damage the machinery or reduce efficiency.

Ensure the torque capacity of the Kelly bar doesn't exceed the rig's torque rating.

And we also should consider safety factor. It's a good practice to apply a safety margin to the estimated torque to ensure the rig can handle unexpected hard layers or obstacles. Typically, a 20-30% safety margin is applied to account for variations in soil resistance.

- Diameter and Compatibility

The kelly bar's diameter should be compatible with both the drilling rig and the borehole specifications.

It should fit the rotary drive unit of the rig, and its sections should match the hole's diameter.

- Material and Durability

Select a kelly bar made from high-strength materials such as hardened steel for durability and resistance to wear in tough conditions.

Unidrill kelly bars use the high quality materials, and are suitable for heavy load, high strength or super big rotary drilling rig. Long-life using and low failure rate.

- Locking System

The locking system (for interlocking bars) should be strong and easy to engage/disengage. This ensures smooth transitions between sections.

- Weight Considerations

The weight of the kelly bar should match the lifting capacity of the rig's crane or winch to avoid overloading and potential damage.

- Maintenance and Spare Parts Availability

Ensure the kelly bar is easy to maintain and that spare parts are readily available to minimize downtime. 

Drillmaster supplies kinds of kelly bar accessories and support customizing kelly bar. All in one solution, provided by Drillmaster.