Views: 0 Author: Site Editor Publish Time: 2026-11-02 Origin: Site
The friction kelly bar transfers torque and drill weight by friction between the splines of each section without rigid locking structures. Its maximum applied pressure is limited by the friction coefficient, and slipping and pressure loss tend to occur in hard formations. The interlocking kelly bar forms a rigid force-transmission structure via engagement and locking of pressure lugs on the bar body. The downforce from the rotary head can be fully delivered to the drill bit with low transmission loss. It can stably supply high drill weight and large torque, making it the key configuration for rock penetration.
Friction kelly bars are suitable for soft to medium-dense formations, including clay, silty soil, sand, backfill, loose cobbles and completely weathered rock. These formations feature low drilling resistance and do not require constant high pressure for rock breaking, so friction bars can perform at their best. Interlocking kelly bars are designed for high-hardness complex formations such as moderately and slightly weathered rock, dense cobbles and hard cemented layers. They excel in rock-socketed pile projects. They can also be used in soft soil, yet are less cost-effective than friction kelly bars.
The friction kelly bar adopts a lighter structure and can be made into 5 to 6 telescopic sections. It can achieve greater drilling depth with the same main machine, ideal for deep soft-soil foundation piles. Due to the load-bearing requirement of locking lugs, the interlocking kelly bar has thicker pipe walls and heavier self-weight, normally with 3 to 4 sections and relatively limited maximum drilling depth, mainly used for medium-deep rock-socketed holes. Multi-section friction bars telescope smoothly with no risk of locking jamming. More sections on interlocking bars mean harder unlocking operations and higher requirements for operator proficiency.
The friction kelly bar is easy to operate. No reverse rotation for unlocking is needed during hoisting and lowering, with continuous smooth telescoping and fast single-bucket cycle, delivering high continuous construction efficiency. It is friendly to new operators and shows prominent advantages in mass pile construction for urban municipal and building projects. Before each hoisting, the interlocking kelly bar requires reverse rotation to unlock sections one by one. Incomplete unlocking may easily cause bar lifting, jamming or damage to locking lugs. It involves more working steps and longer time per bucket, so standardized operation habits are required to prevent damage.
The friction kelly bar has a simple structure with few components. Maintenance mainly focuses on wear of friction splines, featuring low repair frequency and affordable spare parts, bringing lower overall cost for long-term high-turnover projects. Locking lugs and spline blocks of interlocking kelly bars are high-load wearing parts. Under rock penetration conditions with heavy impact loads, regular inspection, surfacing welding and block replacement lead to higher maintenance cost. Its purchase price is also higher than that of friction bars of the same specification. Combined kelly bars are available for projects with alternating formation conditions to adapt to both working scenarios.
Selection shall be based on three factors: geological survey report, pile depth, hole diameter and rig torque. Choose friction kelly bars for pure soft soil, large batch piles and tight construction schedules. Select interlocking kelly bars directly if rock socketing, hard cobbles or continuous high-pressure rock breaking are required. Match the parameters of the rotary head to avoid overload damage caused by fitting interlocking bars on low-torque rigs. Proper kelly bar matching protects the main machine, stabilizes hole-forming quality, cuts downtime for repairs and ensures on-time project delivery.
