Email: sales@unikelly.com WhatsApp: +86 19108462246
Email: sales@unikelly.com WhatsApp: +86 19108462246
The correct Kelly Bar length and number of sections depend primarily on the required drilling depth, rotary drilling rig dimensions, available drilling stroke, mast height, Kelly box position, and drilling method. A deeper hole does not simply require a longer Kelly Bar. The total length, individual section length, and number of telescopic sections must work within the rig's operating range while providing enough effective drilling depth. In most applications, a Kelly Bar may be configured with 3, 4, 5, or 6 sections. The right configuration should be determined from the actual rig and project requirements rather than selecting the maximum possible length or number of sections.
One of the most common mistakes when selecting a Kelly Bar is treating its total extended length as the same as the achievable drilling depth. In practice, the usable drilling depth depends on more than the Kelly Bar itself.
The effective drilling depth is influenced by the Kelly Bar's working length, drilling tool dimensions, rotary head position, mast height, drilling stroke, and the depth at which the drilling tool must operate. The connection between the Kelly Bar and the drilling tool also occupies part of the available working length. For example, a project requiring a 50 m borehole does not necessarily mean that a 50 m Kelly Bar should be specified. The drilling rig must have enough mast height and stroke to deploy the Kelly Bar correctly, while the drilling tool must remain properly engaged at the required depth. This is why Kelly Bar length should always be selected together with the drilling rig's operating dimensions.
A telescopic Kelly Bar consists of several nested sections. When the number of sections increases, the Kelly Bar can achieve a greater extended working length while maintaining a relatively manageable retracted length.
Common configurations include 3-section, 4-section, 5-section, and 6-section Kelly Bars. However, adding sections is not simply a way to increase drilling depth. The number of sections affects several aspects of the drilling system, including the maximum extended length, retracted length, section overlap, weight distribution, stability, and load transferred between sections. More sections may be beneficial when greater drilling depth is required within a limited mast height, but the design becomes more complex and each section must maintain adequate engagement during operation.
For this reason, the number of sections should be selected after determining the required working depth and the physical limitations of the rotary drilling rig.
| Kelly Bar Configuration | Typical Consideration |
| 3 sections | Suitable for relatively moderate drilling depths and applications with sufficient mast/stroke |
| 4 sections | Common configuration for a wide range of foundation drilling applications |
| 5 sections | Useful when greater working depth is required while maintaining a manageable retracted length |
| 6 sections | Used for applications requiring greater telescopic extension and specific rig configurations |
The rotary drilling rig should be the starting point when selecting Kelly Bar length. The most important parameters include the mast height, rotary head travel or drilling stroke, Kelly box position, maximum torque, and available working clearance. A Kelly Bar that is too long may not retract sufficiently within the mast or may interfere with the rig during transportation and operation. A Kelly Bar that is too short may not reach the required drilling depth even when the rig itself has sufficient capacity. The drilling stroke is particularly important. The rotary head needs enough travel to extend the Kelly Bar progressively as the hole becomes deeper. The relationship between the Kelly Bar section lengths and the rig's stroke determines how effectively each section can be deployed. The Kelly Bar must also maintain sufficient overlap between telescopic sections during operation. Excessive extension with insufficient engagement can negatively affect load transfer and stability, so the maximum theoretical length should not be treated as the recommended working length.
Ground conditions should also be considered when determining the Kelly Bar configuration. Soil drilling, mixed ground, weathered rock, and hard rock can impose very different torque and impact loads on the Kelly Bar. For relatively demanding formations, the Kelly Bar may need a stronger structural configuration and an appropriate locking system. An interlocking Kelly Bar provides positive mechanical engagement between sections and is commonly selected for applications requiring higher torque transmission. A friction Kelly Bar relies on friction between sections and can be suitable for drilling conditions within the operating range of the friction system. The drilling tool also matters. A Kelly Bar working with a soil bucket may experience different loading conditions from one operating a rock bucket, core barrel, or heavy-duty auger. Therefore, selecting the number of sections based only on drilling depth can result in an unsuitable configuration.
The objective is to achieve the required depth while maintaining adequate torque transmission, section engagement, structural strength, and operational stability.
A reliable selection process starts with the project requirements and then works back to the Kelly Bar configuration: Required drilling depth → drilling tool → rig dimensions → required working length → section number → section length → Kelly box and connection → final Kelly Bar design. Before ordering a Kelly Bar, provide the manufacturer with:
| Information | Why It Is Required |
| Rotary drilling rig brand and model | Establishes the basic machine configuration |
| Required drilling depth | Determines the target working length |
| Maximum drilling diameter | Helps determine the drilling tool and Kelly Bar configuration |
| Mast height | Defines available vertical working space |
| Drilling stroke | Determines how the Kelly Bar can be progressively extended |
| Maximum rotary torque | Determines structural and locking requirements |
| Kelly box dimensions | Confirms the upper connection |
| Ground conditions | Helps determine the required Kelly Bar type and strength |
| Drilling tool | Determines the operating load and connection requirements |
| Preferred number of sections | Provides a starting point for configuration |
UNIKELLY manufactures telescopic friction and interlocking Kelly Bars for rotary drilling and foundation applications. The Kelly Bar can be configured according to the drilling rig, required depth, torque, drilling diameter, number of sections, and ground conditions. The key principle is that Kelly Bar length and section number should be designed as part of the complete rotary drilling system. A 5-section Kelly Bar is not automatically better than a 4-section Kelly Bar, and a longer Kelly Bar does not necessarily provide better drilling performance. The correct configuration is the one that reaches the required depth while remaining fully compatible with the rig's mast, stroke, rotary drive, drilling tools, and operating conditions.
If you are unsure whether you need a 3-, 4-, 5-, or 6-section Kelly Bar, provide the rig brand and model, required drilling depth, drilling diameter, maximum torque, mast/stroke information, and Kelly box dimensions. UNIKELLY can use these parameters to determine the appropriate Kelly Bar length and section configuration for your application.