Definition - What does Drill Collar mean?
A Drill Collar is a tubular piece of a drill string which provides weight on the bit for the purpose of drilling. While drill pipes make up the majority length of drill strings, drill collars make only a small amount. They have thick walls machined from the solid bars of steel. The steel bars are usually of plain steel but are sometimes of non-magnetic nickel-copper alloy or of other non-magnetic premium alloys. These bars are drilled from end-to-end in order to provide a path for pumping drilling fluid via the collars.
Petropedia explains Drill Collar
Gravity acts on a large mass of the drill collars to offer the downward force required for bits to break rocks efficiently. To control the force applied to bits accurately, a driller has to carefully monitor the surface weight measured when the bit is just off the bottom of a borehole. The drill collars are available in spiral and slick designs with some additional features which can be used for trouble free and safe operations. The spiral collars are designed to help in preventing drill strings from getting stuck in holes by allowing lubricant and drilling debris to pass freely around the drill collars.