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Core Barrels: Understanding Your Head Assembly

posted by Rod McCoremick, Blog author, Fordia Powered by Epiroc

April 9, 2018

In an earlier blog, we looked at the core barrel assembly, this being the equipment that is used to recover core, and plays a very important role in the diamond drilling operation. The success of an operation depends on retrieving core efficiently and effectively. We looked at the parts of the inner tube of which the head assembly is a part. It is a complex part so it is well worth looking at it in detail.

Fordia Coring System 2018

Drillers and helpers alike should have a thorough understanding of the function of each part of their equipment. Let’s take a look at the parts and functions of the head assembly.


Spearhead assembly:

This is the pointed and machine-hardened tip of the head assembly. When retrieving the inner tube assembly, the overshot connects and locks onto the spearhead. The knucklehead function also allows for the entire inner tube assembly to be turned by 90 degrees when lowering out of the mast.


Latch retracting case:

The latch retracting case covers the latch body and the mechanism that enables latching. The latch retracting case is connected to the upper latch body as well as the spear head base by means of a spring pin or coiled pin.  When the overshot pulls up on the spear head, the latches retract allowing the Inner tube assembly to release from the outer tube when the core sample gets extracted.



When inserted into the outer tube, the latches extend outward and lock into within the machined recess of the adaptor coupling. The hardened bottom end of the locking coupling holds the laches in the outer tube, keeping the inner tube assembly securely in place. There are several options when choosing latching systems such as the traditional spring latch, Fordia’s V-latch and L-latch, for example.


Latch body:

The latch body contains water ports around the landing shoulder (on the head assembly) that allow water to bypass the landing ring (on the outer tube) on the way to the core bit. The upper latch body provides the pivot for the latches. The lower latch body is threaded onto the upper latch body and holds the landing shoulder in position. The lower latch body accepts the upper portion of the spindle, allowing for adjustment of the inner tube length. It houses the landing indicator bushing and steel ball.


Landing indicator bushing and ball:  

The landing indicator is found within the latch body. Before the inner tube is pumped into place, the helper will force the steel ball into position, popping the ball just above the bushing. When the inner tube assembly lands in place on the landing ring, pressure rises as the drill string fills with water.  Once completely full, the water pressure forces the ball through the nylon bushing causing a spike in pressure visible to the driller on his water gauge, giving him an indication that his inner tube is in place, latched, ready to drill.


Landing shoulder:

The landing shoulder is a heat-treated ring between the upper and lower latch bodies that makes contact or “lands” on the landing ring found in the outer tube assembly. It prevents the inner tube assembly from moving through the outer tube assembly. It creates a barrier that forces the drilling fluids to go through the water ports on the upper and lower latch bodies and gives the pressure signal mentioned above.


Spindle shaft:

This piece connects the upper and lower parts of the head assembly. The spindle bearing assembly allows the head to spin freely and the inner tube to remain fixed while coring. It allows the driller to adjust the length of the inner tube assembly to maintain the proper adjustment between the core lifter case and the bit bevel. The shut off valves, washers, bearing assembly, compression spring and lock nut are all mounted to the spindle.


Shut off valves:

These are rubber or nylon discs found on the spindle shaft. When there is a core block, or the tube is full, these discs are compressed. As they compress, they increase in diameter and come into contact with the outer tube and will restrict or cut off the flow of water. This registers on a gauge on the surface as a positive peak in pressure and alerts the driller. There are two types of shut off valves: hard and soft. Hard shut off valves would be used for competent core and deeper holes, while soft ones would be used for drilling fragile shallow formations.


Valve adjusting washers:

These two metal washers separate the nylon or rubber shut off valves allowing for added reliability.


Thrust Bearing:

This is located just below the lowest valve adjusting washer and absorbs the thrust force when the tube is coring, allowing the spindle bearing/bushing to function properly.


Spindle bushing:

The spindle bushing (or bearing) allows for a rotating string and a fixed core to coexist happily. It threads onto the inner tube cap.


Compression spring:

When retrieving core, the drill string is pulled back and the spring is compressed. This allows the core lifter case to rest on the inside bevel of the core bit. This transfers the load required to break the core, from the inner tube assembly to the core bit.


Inner tube cap:

The inner tube cap threads onto the spindle bearing and houses the lower portion of the spindle, the spindle bearing, compression spring and lock nut. A grease fitting recessed into the side, delivers grease to the vital moving parts. The bottom end of the inner tube C-cap also houses the check valve which acts as a one-way valve.


So there you have it – understanding your equipment can help improve drilling performance and helps you with troubleshooting if problems arise. Most importantly, it can reduce safety risks. We’ll look at the outer tube components in an upcoming blog. Meanwhile, if you have any questions about parts or their function, you can always contact our technical field support team for advice.