Understanding Flange Couplings

A flange coupling is a type of rigid coupling used to connect rotating shafts so that they act as one unit. This allows power and torque to be transmitted from one shaft to the other. It consists of two flanges, one mounted on each shaft end, which are then bolted together.

Role of Bolts in Flange Couplings

In a flange coupling, the bolts play a critical role not only in holding the two flanges together but also in transmitting the torque from one flange to the other. This torque transmission can happen primarily through two mechanisms:

• Friction between the flange faces due to the clamping force of the bolts.
• Shear resistance of the bolts themselves, acting as dowel pins to prevent relative rotation between the flanges.

For the bolts to effectively resist shear forces and prevent relative slip between the flanges under load, they need to fit snugly into the holes in the flanges.

Why Reamed Holes are Used for Bolts

When bolts are intended to carry shear loads directly (as in transmitting torque in a flange coupling), the holes in the connected parts are often finished precisely. This is where the term 'reamed holes' comes in.

Reaming is a machining process that enlarges and finishes a hole to a precise size and creates a smooth internal surface. Holes that are simply drilled may not be perfectly round, straight, or sized accurately enough for applications requiring a tight fit.

Using reamed holes allows for the use of 'fitted bolts' or 'body-bound bolts'. These bolts have a shank diameter that is manufactured to a close tolerance and matches the reamed hole diameter. When these fitted bolts are inserted, they have a snug fit within the holes. This snug fit ensures that any shear load causing a tendency for relative movement between the flanges is immediately taken up by the shear strength of the bolt body, minimizing or eliminating slip.

Comparison with Other Options

• Machined holes: This is a very general term. While reaming is a type of machining, simply stating "machined holes" doesn't guarantee the precision needed for a fitted bolt application designed to take shear loads effectively. Drilled holes are also machined but typically lack the required precision for a tight fit.
• Threaded holes: Threaded holes are designed to receive the threaded part of a bolt to create a fastening, often in conjunction with a nut on the other side. Threading both flanges and using bolts without nuts would be unusual for a standard flange coupling and would not provide the necessary shear strength transmission through the bolt body itself across the joint.
• Gasketed holes: Gasketed holes refer to holes in components that are part of a sealed joint, where a gasket is placed between the faces to prevent leakage. The presence of a gasket is for sealing purposes and does not dictate the precision required for the bolt fit to carry shear loads for torque transmission in a coupling.

Therefore, for a flange coupling where the bolts are designed to withstand shear forces and prevent slip for effective torque transmission, the holes are typically finished by reaming to achieve the necessary precise fit for the bolts.