Axial thrust is a force that acts along and parallel to the axis of a rotating shaft. In pumps, compressors, turbines, and gearboxes, this force pushes the rotor toward one end of the machine, loading the bearings that hold it in place. If it isn’t properly managed, axial thrust can destroy bearings, accelerate wear on internal parts, and shut down equipment. Understanding where it comes from and how engineers control it matters for anyone working with rotating machinery.
How Axial Thrust Differs From Radial Thrust
Rotating machines experience forces in two main directions. Axial thrust acts parallel to the shaft, pushing along its length like a spear. Radial thrust acts perpendicular to the shaft, pushing sideways against it. The two forces come from different sources and wear on different components.
Radial thrust typically arises from uneven pressure distribution around the casing of a pump or compressor, especially in single-volute designs. It stresses radial bearings and can cause shaft deflection. Axial thrust, by contrast, is driven by pressure differences between the suction (inlet) side and the discharge (outlet) side of an impeller. Impeller geometry and operating conditions are the primary factors. Each type of force requires its own bearing arrangement and its own design strategy to keep the machine running reliably.
What Generates Axial Thrust
Axial thrust has multiple sources that add together. In a centrifugal compressor, the total axial force on the rotor is the sum of four components: the force from changes in gas momentum as it enters and exits the impeller, the force from pressure differences across the impeller’s front and back surfaces, the force across any balance piston, and the force from coupling pre-stretch if the machine is connected to a driver.
Momentum Change
When fluid enters an impeller, it has a certain speed along the shaft axis. As it passes through the impeller, its direction and speed change. That change in momentum exerts a reaction force on the rotor. In many centrifugal designs, fluid exits the impeller radially (sideways), meaning its axial velocity drops to zero. The momentum force then pushes the rotor toward the discharge end of the machine.
Pressure Imbalance Across the Impeller
This is usually the largest contributor. The back side (hub) and front side (shroud) of an impeller are exposed to different pressures. At the impeller outlet, pressure is high. At the inlet, pressure is low. Because the hub and shroud have different exposed areas, the pressure forces on the two sides don’t cancel out, and the difference creates a net push along the shaft. The exact magnitude depends on how pressure varies across these surfaces, which is influenced by the spinning fluid in the narrow gaps between the impeller and the casing.
In multistage pumps, this effect is amplified. Fluid leaking through the small clearances around each impeller stage creates complex flow patterns inside the pump cavities. On the front side of an impeller, leakage flows inward toward the shaft, while on the back side it flows outward. The spinning speed of the fluid in these cavities, relative to the impeller speed, directly controls how much axial force each cavity generates. When the fluid in a cavity rotates faster than half the impeller speed, axial force increases. When it rotates slower, axial force decreases.
Helical Gears
Axial thrust isn’t limited to pumps and compressors. Helical gears, which have angled teeth for smoother and quieter operation, generate axial thrust as a byproduct of their geometry. The force pushes along the gear shaft, and its magnitude grows with the helix angle. The direction depends on which way the gear rotates and whether the teeth spiral left or right. When helix angles exceed about 20 degrees, engineers often use double helical or herringbone gears, which have opposing tooth angles that cancel the axial forces against each other.
How Engineers Calculate Axial Thrust
The calculation starts by identifying each contributing force and summing them. For a centrifugal compressor, the momentum component equals the mass flow rate multiplied by the change in axial velocity between the inlet and outlet. If the gas exits radially, the outlet velocity term drops out, and the force simply equals the mass flow rate times the inlet axial velocity, directed toward the discharge end.
The pressure component requires integrating the pressure distribution over every rotor surface exposed to the gas. Engineers define three key areas: the inlet area, the hub (back) area, and the shroud (front) area. A simplified first-pass calculation assumes constant pressure at the impeller inlet and outlet, then multiplies each pressure by its corresponding surface area. The difference between the hub and shroud pressure forces gives a rough estimate. A more refined calculation accounts for how pressure actually varies across these surfaces due to the spinning fluid in the gaps, which follows predictable patterns governed by the fluid’s rotational speed and the geometry of the cavity.
For helical gears, the axial thrust load is calculated from the transmitted load and the helix angle using straightforward trigonometry. A steeper helix angle means a larger fraction of the transmitted force converts into axial push.
What Happens When Axial Thrust Isn’t Controlled
Unmanaged axial thrust overloads thrust bearings, generating excess heat and dramatically shortening bearing life. The symptoms are predictable: bearings run hot, fail on a regular basis, and the internal wetted parts of the machine wear out faster than expected. In severe cases, the rotor can physically shift far enough to contact stationary parts, causing catastrophic damage. Even moderate excess thrust degrades seals and increases vibration, creating maintenance problems that compound over time.
Thrust Bearings: The First Line of Defense
Thrust bearings are specifically designed to absorb axial loads and keep the rotor positioned correctly. Several types exist, each suited to different load ranges and speeds.
- Ball thrust bearings use balls running between two flat races. They handle moderate axial loads and are common in lighter-duty equipment.
- Tapered roller thrust bearings use small cone-shaped rollers arranged so their axes converge at a single point. The larger contact area between rollers and races lets them carry significantly more thrust than ball types, which is why they’re the standard choice in automotive wheel assemblies, typically installed in pairs to handle thrust in both directions along with radial loads.
- Tilting-pad (hydrodynamic) thrust bearings use a thin film of oil between pivoting pads and a rotating collar. They excel in high-speed, high-load industrial machines like compressors and large pumps. Industry standards for centrifugal pumps require hydrodynamic radial and thrust bearings when the product of rated power and rated speed exceeds a specific energy density threshold, roughly 4 million kilowatt-revolutions per minute.
For pipeline pump services, the latest edition of the API 610 standard allows ball thrust bearings paired with hydrodynamic radial bearings even above that threshold, provided the manufacturer has successful field experience. Some installations have operated reliably at energy densities nearly three times the original limit. For new designs, finite element analysis of the bearing housing helps determine which bearing arrangement can handle the expected loads.
Reducing Axial Thrust by Design
Relying on thrust bearings alone isn’t always practical, especially in multistage machines where the cumulative axial force across several impellers can be enormous. Engineers use several design strategies to reduce thrust before it reaches the bearings.
Balance Holes
Drilling small holes through the impeller hub connects the high-pressure cavity behind the impeller with the lower-pressure main flow path. This drops the static pressure across the entire back surface of the impeller disc, reducing the pressure imbalance that creates axial force. The holes introduce a highly swirling flow pattern in the gap behind the impeller, which lowers the effective pressure over that surface. The tradeoff is a small amount of internal leakage, which slightly reduces the machine’s hydraulic efficiency.
Balance Drums and Discs
A balance drum (also called a balance piston) is a cylindrical sleeve mounted on the shaft that creates a controlled pressure drop to oppose the net axial thrust. High-pressure fluid on one side and low-pressure fluid on the other generate a counteracting force. Balance discs work on a similar principle but use a flat disc and a small axial clearance that self-adjusts: if the rotor shifts, the clearance changes, the pressure balance shifts, and the rotor is pushed back into position. Both approaches are common in multistage centrifugal pumps and compressors, though long balance pistons can sometimes introduce rotordynamic instability if not carefully designed.
Opposed Impeller Arrangements
In multistage pumps and compressors, mounting some impellers facing one direction and others facing the opposite direction causes their individual thrust forces to partially cancel. This “back-to-back” arrangement reduces the net thrust that the bearings must absorb, though it complicates the internal flow path and casing design.
In practice, most machines use a combination of these approaches. A multistage compressor might use back-to-back staging to cut the gross thrust in half, a balance piston to absorb most of what remains, and a tilting-pad thrust bearing to handle the residual force and any transient load spikes during startup or off-design operation.