Marcus Chen is a project engineer. Last year, he was doing a 360 kN·m rig installation in downtown Shenzhen, on a high-rise structure. It was therefore no surprise when he ordered a 508mm kelly bar for that specific rig. In high-rise foundation work, one does not expect the rig to max more than the bar rated 360 kN·m. It was as if someone was predicting the future!
A few months later, the bar broke just after it had advanced to a height of 42 meters. At optimal loading capacity, the main drive keys twisted off. The repair work cost 18,000 dollars. The bonus law on the project was canceled due to the lateness of the completion time.
As often happens in the field of foundation drilling, Marcus was guilty of an error. He took the Kelly bar torque specification, which was found on the datasheet and equated it to the torque that the Kelly bar could effectively transmit under real torque conditions. Kelley bar torque engineering and usable torque are not mere technicalities. The absence of that knowledge leads to a great level of efficiency of rebar systems breaking down significantly and, in most cases, ending before they could have been wound up.
In the following passage, the torque ratings of Kelly bars shall be discussed. Particularly, how the torque is transformed from rig to tool and the selection of bar capacity for the rotary drilling rig the torque output of which is to be considered for safety measures. How to study specification tables, obtaining the usable torque for a particular kind of soil and determine the most appropriate Kelly’s diameter for the available rig.
What Is a Kelly Bar Torque Rating?
The Kelly bar torque rating can be considered the highest angle of rotation that it can exert without any breach. This is denoted in kilonewton meters (kN·m) by the manufacturers and represents the maximum effect that the kelly bar can offer before a breakage is experienced.
However, one has to understand that the values of the Kelly bar torque capacity in the specification sheet do not represent the level of torque that is subjected to the drilling tools. Several factors are at play to reduce the force given to the soil before even applying it. This knowledge is the basis of selecting bars intelligently.
A rotary drive of the assembly is the starting point of a torque transmission chain. This path also includes sections of the Kelly box bar, drive keys and telescopic sections which ultimately arrive at the tool used at the bottom of a borehole. Every time there’s torque in the system, a fraction of it disappears due to bearing, friction, inefficiency among other physical constraints. Soberly again, it is acknowledged that the torque that will be available for rotation of a tool is always less than the maximum torque that can be generated by the rig.
For contractors new to torque matching, our complete Kelly bar guide provides a broader overview of bar types, materials, and selection basics.
How Torque Travels from Rig to Tool
Torque does not indicate extensively that from a rotary head equipped with the presented design would reach a hole at the same depth as a bucket excavated by an inclined tubewell. It is mostly transmitted through a mechanical chain, barring for a fraction of its splitting and bearing perspective.
The Torque Transmission Path
Events in the chain easily fit together: the prime mover does its work and puts the torque into the kelly box, turning it. This Kelly box, externally, contains drive keys within the last section of the bar. Torque flow circles by the box and keys, and this torque is either in contact with the inner sections only, or is locked into them by special mechanical locks. The innermost section is the one that generates and transfers the rotary motion to the operating drilling tool.
At every point in the system, energy dissipates. Particularly when the Kelly boxleacnk and ankle box drives are worn out, there are. The joints having more than one segment create complications for sliding surfaces. In the process of rotary drilling, the tool requires some energy in order of resist turning at different depths of penetration. Because of these forces, the energy required for efficient conditions thus the output, which is the productivity of the rig decreases with these forces.
Why Usable Torque Is Always Lower
The usable torque is that fraction lost. Thus, a 400 kN·m rated rig should yield only a perfect 340–360 kN·m of torque under very idealistic tool conditions. A force component alone could account for 20-30% soil resistance in hard rock or solid gravel materials.
This is the reason why configuring a Kelly bar with the same Kelly bar torque as being a bar is extremely risky. Inasmuch as the bar has already hit its upper curtail transcending the point before the effect of soil resistance is taken into account. Additionally, any high load due to operational spikes or tool biting will not be much of a concern. Such load spikes or tool bites will easily buckle the Kelly bar with break trip of bearings.
Kelly Bar Torque Capacity by Type
It is important to observe that different types of kelly bars relate to resistance to torque differently. The torque limits a specific kelly bar can withstand, as well as its ease of transfer through the various sections, are all dependent on the type.
Friction Kelly Bars
Friction Kelly bars use the frictional force between the grooving of the drive keys and the bore inner surface so that torque is transmitted. They may be thinner and still drill deeper as compared to close-fit models. Nevertheless, the ability of the Kelly bar to carry torque is generally slightly better than this design.
An excellent range of torque for the friction Kelly bar can be obtained, ranging from 120 kN·m for a bar of 299 mm diameter to 360 kN·m for a bar of 508 mm diameter. These Kelly bars are most effectively used in geotechnical drilling on soft and loose soils up to the limit torque. In particularly hard geological conditions, friction torque rods have been known to fail under heavy loading and as a result the drill string or casing is not moved forward by the rods but rather the stress from the slip surface affects the hole.
Interlocking Kelly Bars
One of the other explanations is the employment of interlocking bars which have locking apparatus engaging in every hoop formation made. In this case, when this latch sticks, the connection is relatively straight with less or very negligible power loss. The rating of interlocking kelly bar torque is generally higher than that of the friction kelly bar of the same size.
Interlocking bars can have torsional demand that varies from lower, 180 kN·m, in thin diameter section to as high as 500 kN·m on a 630 mm heavy-duty bar. The degree of effectiveness in maximum load cases is up to 92% owing to the fact that there are no loose connections within the setup. This fact contributes to the situation where for stiff rock, for edging with soil, or deep drilling with the need for precise alignment, such technique is used without any doubt.
Full-Lock Kelly Bars
It should be noted that “full-lock” bars lie on the other end of the torque spectrum. No angle is left unsealed as each bar header gets mechanically engaged with the preceding helical part. For the largest borestones, the maximum torque can exceed 380 kN·m for a default engagement and reach as much as 500 kN·m in the largest bores.
These are produced for the most arduous situations. The kind of rocks that are likely to survive harsh conditions, i.e., granite, basalt, and large boulders, need well-tuned structural stability compared to any other types of Kelly bar; the full-lock designs are the only ones that can provide.
If you choose a Kelly Bar based on your needs, you can learn about it through our article about How to Choose a Kelly Bar.
| Bar Type | Torque Range (kN·m) | Efficiency | Best For |
|---|---|---|---|
| Friction | 120 – 360 | ~78% | Soft to medium soils |
| Interlocking | 180 – 500 | ~92% | Hard soils, rock, deep drilling |
| Full-Lock | 380 – 500+ | ~92%+ | Granite, basalt, maximum stability |
Kelly Bar Torque Capacity by Diameter
Torque capacity correlates directly with the bar’s outer dimensions. When comparing bars of different sizes, the larger diameter bar will be able to exert a higher torque capacity as a larger bar has bigger resistance shapes against torsional stress, therefore has larger radial drive keys for torque transfer, and stiffer bending under load.
Specification Matrix
The capacity normally changes with outer diameter, as set out in the bar torque capacity chart. These values signify a value that can be expected or guaranteed for the typical Kelly bar design. Do not take these values as accurate because it is necessary to verify the exact torque levels which are specific and refer to the type of kelly bar.
| Outer Diameter (mm) | Nominal Torque (kN·m) | Sections | Max Depth (m) | Kelly Box (mm) | Typical Bar Type |
|---|---|---|---|---|---|
| 299 | 140 | 3 | 15 – 43 | 200×200 | Friction / Interlocking |
| 355 | 150 | 3 | 12 – 48 | 200×200 | Friction |
| 394 | 260 | 3 – 4 | 21 – 64 | 200×200 | Interlocking |
| 419 | 280 | 3 – 4 | 24 – 68 | 200×200 | Interlocking |
| 470 | 280 – 360 | 3 – 4 | 24 – 64 | 200×200 | Interlocking |
| 508 | 360 | 3 – 5 | 25 – 53 | 250×250 | Interlocking |
| 558 | 480 | 4 | 30 – 72 | 250×250 | Interlocking |
| 580 | 400 | 4 – 5 | 30 – 78 | 250×250 | Interlocking |
| 630 | 500 | 4 | 30 – 82 | 250×250 | Interlocking / Full-Lock |
Material and Wall Thickness Impact
Another factor that has an impact on Kelly bar torque capacity is the steel grade and the wall thickness of the bar. Most bars with wall thicknesses between 18 and 22mm are composed of 35CrMo homogenized steel. Concrete-built in situ steel bars, which are mainly used in flat land and involve grout-bonded bars, Q460D or grade Q550 steel with thickness up to 25 – 40mm, are some examples of these.
With a thicker wall, there is an increase in the bar’s ability to resist torsion without altering its outer diameter. For example a 419 mm bar with 32 mm thick wall can handle a bigger torque than if the same length bar were to have a 20 mm thick wall. When examining different bars, the diameter and wall fractions should be taken into account.
During a piling installation work that was performed in Malaysia, some hard lessons had to be learnt by Arif Rahman, the concerned contractor. Arif saw that a 419 mm bar was in bad shape and purchased a marginal bar of a different dimension instead; the walls were very thin. Within just 800 operating hours under medium-hard limestone conditions the new bar broke. The original had 28 mm thick walls to withstand torque loads of the site. The replacement consisted of 20 mm max thick walls only. In conclusion, it was decided to deal soon with the weakened lattice faces rather than deal with the thicker interlocking bars. Such lower walls were inappropriate for the required torsional stress. And Arif now inspects wall thickness each time he checks the torque rating of a kelly bar before purchase.
To choose the right size for the Kelly box, please refer to our article on Kelly Box Sizes.
Calculating Usable Torque for Your Job
Spec data sheets are really helpful when seeking to fit a bar to a scaffold. So, much more than simply matching two numbers, it requires the exertion of such effort into the exact calculation of what can be referred to as the usable torque, such as will be able to propagate to the mounted equipment, given the conditions of the work place.
The Usable Torque Formula
A practical approach to estimating usable torque looks like this:
Usable Torque = Rig Max Torque × Drive Efficiency × Soil Factor × Wear Factor
Each factor accounts for a different source of torque loss.
The kelly box, drive keys, and the telescopic joints are almost like gears, where the drive efficiency plays a vital role. A fully functional system can deliver up to 90% to 95% efficiency. If these components are old or their lubrication is nothing but a memory, they would most probably operate at 80%.
Soil factor is an element determining the amount of resistance that the tool is subject to. A resistantless environment like soft clay has factor close to 0.95. The resistance of a medium-thick layer of gravel increases the factor to 0.75. In heavy solid rock this factor could decrease below 0.70 since the tool has to break up the medium and not merely dislodge it.
Wear factor is a bar-specific factor that is a measure of the rate of wear of the bar. A new fresh bar is at an average operation point of 1.00. A bar with worn driver keys, thin walls on the bar, or microcracks develops below the 0.85 level.
Worked Example
Assume that the restraint of the rig is 360 kN·m maximum. Further assume that you are using a 508 mm interlocking bar whose kelly bar torque rating is 360 kN·m as well. The drive system is quite decent. The soil is dense gravel and boulders. The tenure of the bar has spanned 1200 hours.
- Rig max torque: 360 kN·m
- Drive efficiency: 0.92 (well maintained)
- Soil factor: 0.75 (dense gravel)
- Wear factor: 0.90 (1,200 hours of moderate wear)
Usable torque = 360 × 0.92 × 0.75 × 0.90 = 223 kN·m
The torque applicable to the tool is 223 kN·m, which is much lower than the one mentioned above. But, here’s the thing, in order for the rig to get over an obstruction, should there be a dorsal rate torque, the bar instantly becomes 360 kN·m. The Kelly bar torque rating of the machine is 360 kN·m. So, that peak without a safety factor stresses the bar the most. There is a possibility of fatigue every time there are spikes.
This is why the Kelly bar safety factor matters.
Why the Kelly Bar Safety Factor Protects Your Investment
A safety factor for Kelly bars describes how much higher their nominal capacity is in comparison to the amount of torque output a drilling rig is capable of producing. According to recommendations, the standard safety factors are usually set between 1.2 and 1.5 times the rated capacity. What this means is that the capacity of the Kelly bar in terms of torque must exceed the output of the drilling rig by at least 20% to 50%.
How Safety Factors Work
Having a safety cap of 1.2 times, a kelly rig with consequent torque can only be maintained with A 360 kN·m rig as the figure will be supported by a 440 kN·m kelly bar that being indicates that the 440 mm length kelly set up will be rated at 480 kN·m, and will require a 558 mm in the 480 kN·m nominal system With a 1.5 times safety cap, however, the overall requirement is 540 kN·m, which would require a 630 mm bar of the 500 kN·m rating or a modified section.
The issue of risk is not lack of engineering conservatism. It occupies a middle place in practical risk management systems. Earth formations may change. Equipment may fail. Where the packer does not open an accessory and full torque is applied to the bail trying to recover a sand bucket. Given a razor-thin line, all accidents still become potential errors.
Hard Rock Demands Higher Margins
Hard rock and abrasive soils cause enormous torque demands and a quicker wear rate. In such cases, you need to adjust your Kelly bar safety factor towards the higher end of 1.5x. The higher torque perturbations and the presence of hard or abrasive particles typical of rock drilling lead to application of the most severe loads on the kelly bars as compared to the operation of the soft subsoil.
Warning Signs of Insufficient Margin
If the bar is inadequately strong, there will be an alert before any catastrophic failure occurs. Drive keys burn out, causing sharp edges and fine cracks to extend too. Torsional twist is visible in the sections. Finally, the bar is heavily vibrating at maximum load. As the wear intensifies, the maintenance period will shorten.
Grab your attention. If that is the case, then it refers to the accuracy of selection of the Kelly bars, in particular, the correctness of their installation and the rating of the torque they can sustain based on the given rig’s maximum output.
Matching Your Rig to the Right Kelly Bar
Choosing an appropriate Kelly bar requires only a simple fives-step process. Practice will help you strictly adhere to the above much better and prevent premature malfunction due to carelessness.
Step 1: Identify Your Rig’s Maximum Torque Output
Check your rig’s technical data sheet for the rotary drive maximum torque. Common ranges by manufacturer include:
- Bauer BG series: 240 to 420 kN·m
- Liebherr LB series: 180 to 400+ kN·m
- Sany SR series: 200 to 360 kN·m
- Soilmec SR series: 160 to 280 kN·m
- XCMG XR series: 180 to 360 kN·m
Step 2: Apply the Kelly Bar Safety Factor
Enhance the peak rotational yield of your rig by a certain number of times. If damaged mixing is being carried out, with a general value of 1.2x arc. In the case of dense rock/core sampling, a value of 1.5x can be considered a rudimentary level still mentioning the presence of.
Step 3: Match Diameter to Torque Requirement
Using the torque chart for Kelly bars, figure out which is the smallest number of the bar given which should achieve the given rating or exceed it. If one needs 480 kN·m, the standard bar is 558 mm and in case of 540 kN·m, 630 mm bars are the next size.
Step 4: Verify Kelly Box Compatibility
The Kelly box is the point from which the bar is connected to the drilling machine. Typical dimensions of these boxes are usually between 130×130 mm and 250×250 mm. A kelly box that is not properly fitted will cause leaks and unnecessary pressure on the bar and the drive unit which will lead to excessive damage.
Step 5: Confirm Depth and Soil Requirements
Adjust the number of segments of the bar to meet the requirements. With a four-segment bar of 558 mm diameter, approximately 72m can be reached. The configuration with a five-segment bar of 508 mm reaches a similar depth but has lower kelly bar torque capacity. Select the systems bearing in mind the required depth, torque as well as the task cost effectiveness.
If you are unsure which bar type suits your ground conditions, our friction vs interlocking Kelly bar comparison explains the mechanical differences and their practical impact on torque transfer.
What Happens When Torque Is Mismatched
Regarding torque mismatch, it is not only peculiar to the larger bars. A bar that is smaller than it should be, when subjected to any form of stress, buckles. On the other hand, oversized bars are essentially non-useful as they have excess capacity and hence a waste in terms of time and money.
Under-Specification: The Bar Is Too Small
When the kelly bar torque rating is lower than the crane output, the bar becomes the weakest link. Thus, force shatters retention elements. Sections fail due to repeated bending. There are even situations when the load creates cracks at some points where the structure is reinforced. In some of the cases, nobody can protect from the breakdown, which is sharp and destructive, the jamming of the stem in the borehole or tear at the connection.
Cost overruns are not limited to the bar alone. This assistant provides rent money to hoisting rig operators. But attention, better to angle the cross bore. Every hour of operation will cost us several thousand more dollars. Do not take for example, the use of a 1 size smaller bar when it comes to a single failed torque application.
Over-Specification: The Bar Is Too Large
The magnified bar will not cause harm but it will be useless. Such torque applied is overkill to the rig’s structure for a 200 kN·m capacity piling rig with a 630 mm diameter bar. It will no longer be easy to push and maneuver the bar up and extends the time which takes to place the bar at its designated position. The amount of fuel consumed is affected. If the ground is soft, there is also a possibility that the penetration rate can decrease because the fast spinning of the barrel will hinder rotation.
The goal is to match the bar to the job, not to buy the largest bar that fits the rig.
Torque Monitoring and Maintenance
There are practices in the current era of complex drilling operations that have most effectively increased the life of the rods, mainly the monitoring and upkeep of the history of the loads which the rods are subjected.
Wireless Torque Monitoring
Real-time data on load of the applied torque can be obtained by means of wireless torque sensors, which are set between the rotary drive and the kelly box. This technique provides the user with the latest information. It shows when the high loads are observed, when the rebar works near the Kelly bar load, and changing soil conditions are forecasted.
Connecting Torque to Maintenance Schedules
Bars designed to operate in or near the higher end of their Kelly bar torque ratings face more wear and tear compared to the ones which operate at relatively lower loads. At the moment you notice that your torque data indicates an increased torque, it’s best that you augment the recommended optical settings. To be more specific, instead of checking the guard lock down lugs every 250 hours, consider checking them every 150 hours. Out of them all and section alignment should be monitored during the operations.
For detailed maintenance protocols, our Kelly bar maintenance guide covers inspection schedules, cleaning procedures, and component replacement criteria.
Conclusion
A Kelly bar torque designation is not just numerical information placed in a list of product values. It serves as a safety measure to guard your material, your program, and your team. Recognizing the distinctions between the rated and the allowable torque values is critical in selecting the correct tool.
Begin with the highest rated value given for the maximum torque output of your unit. Increase this value by applying a Kelly bar safety factor of 1.2x to 1.5x depending on the ground where your crane goes. Make use of the Kelly bar torque table to calculate the diameter that matches the torque load. Confirm the specifications of the Kelly bar kelly box. Next, measure the actual torque loads on the ground and reschedule the maintenance intervals at the positions.
The field rewards are evident. A properly selected rod is used in the relevant conditions. It has fewer repairs, longer service life, and, therefore, less idle time. That is why commercial drilling is mostly lost: one world in particular.
If you are unable to determine how the kelly bar may be related to the moment of resistance of the machine, you can simply make a call to Changsha Mingyi Machinery Equipment Co., Ltd. Our trained staff involved in engineering work will be able to discuss your machinery data, soil conditions as well as the work being done in order to come up with dimensions of bar, type and components that are suitable for such conditions.
We offer tailor-made Kelly bars complete with proven torque capacity as per the standards of the relevant manufacturers.
Contact us today for expert torque matching support and custom Kelly bar specifications.
