How to adjust the axial clearance of a multistage pump

Axial clearance (also called axial movement) refers to the minute distance that the pump rotor (impeller, shaft, etc.) can move axially. Adjusting the axial clearance of a multistage pump is a crucial step in ensuring normal operation and preventing wear and seizure. Axial clearance determines the compensating thermal expansion of the pump shaft; this distance cannot be too large or too small. If the axial clearance is too small, it will cause difficulty in bearing rotation, rolling element jamming or damage. If the clearance is too large, it will lead to severe vibration or cage damage. When adjusting the axial clearance, we need to determine the starting point, calculate the adjustment amount, and then adjust the axial clearance twice, conducting continuous testing to ensure that the axial clearance will not affect normal operation. Next, HNYB will introduce the method for adjusting the axial clearance of a multistage pump.

 

 

• Method 1: Use the adjusting nut on the thrust bearing body (most common)

This method involves tightening or loosening the round nut on the bearing housing to drive the entire rotor assembly axially.

 

Preparation: Prepare the following tools: wrench, screwdriver, feeler gauge, vernier caliper, dial indicator (recommended for precise measurement), copper rod, and hammer. Ensure the pump is powered off and isolated, and the medium is drained. Remove the coupling cover, mark the coupling alignment position, and then disconnect it.

 

1. Measure the Initial Total Runout

Remove the rear bearing cover of the pump to expose the adjusting nut.

Install a dial indicator on the shaft end, with the needle pointing towards the shaft end face.

Without the bearing installed, push the shaft forward and backward, recording the maximum and minimum readings of the dial indicator. The difference is the total runout. This value is inherent to the pump design and is very important.

 

2. Determine the Target Operating Runout

The operating runout is usually about half of the total runout. Refer to the pump manufacturer's technical manual for specific values. If no manual is available, an empirical value is 0.10mm ~ 0.30mm.

The goal is to adjust the rotor to the middle of the total runout, allowing for the operating runout. For example, if the total runout is 1.0mm and the target operating runout is 0.2mm, then the rotor should be positioned to move backward by (1.0 - 0.2)/2 = 0.4mm from its initial position.

 

3. Adjustment Operation

Positioning: Push the rotor towards the pump's drive end (coupling end) until it is in its forward position. At this point, the impeller may be in contact with the pump casing.

Installing and Pre-tightening the Bearing: Install the thrust-side bearing (e.g., angular contact ball bearing) and initially tighten the adjusting nut to eliminate bearing clearance. The dial indicator reading should be zero or a fixed value.

Adjusting Backward: Slowly and evenly rotate the adjusting nut, pulling the entire rotor assembly towards the non-drive end (rear end). Observe the dial indicator until the pointer shows a predetermined movement (e.g., 0.4mm in the example above).

Locking: After adjustment, secure the adjusting nut firmly with a locking shim or another locking nut to prevent loosening during operation.

Verification: Gently push the shaft back and forth again to check if the dial indicator reading of the working runout meets the requirements (0.10~0.30mm). If not, loosen and readjust.

Manually rotate the rotor to check if the rotation is flexible and even, without friction or jamming.

Reset: Install the bearing cover and coupling, re-check the alignment, and finally install all protective covers.

 

• Method 2: Adjust using the shims between the pump body inlet ring and the impeller.

In some older or smaller pumps, the axial clearance is controlled by the thickness of the gasket between the impeller hub and the pump body.

 

Disassembly: Remove the entire rotor assembly from the pump body.

Measurement: Measure the axial overlap distance between the impeller inlet ring and the pump body inlet ring.

Adjustment: Change the axial position of the impeller on the shaft by increasing or decreasing the thickness of the adjusting shims between the impeller rear hub and the bearing housing.

Installation Verification: Reinstall the rotor, without tightening the rear end cover bolts, manually rotate the pump to check if the impeller is centered in the volute. This can be aided by measuring the clearance between the impeller on both sides and the pump casing. After confirmation, tighten all bolts and rotate the pump again to confirm smooth rotation.

Refer to the Manual First: Different manufacturers and models of pumps may have slight differences in design runout and adjustment methods. The reliable method is to strictly follow the original manufacturer's manual for the pump.

Measuring Tools: A dial indicator is strongly recommended for accurate measurement. Relying on feel or feeler gauges has a large margin of error.

Even Tightening: When tightening the adjusting nut, use a dedicated wrench and apply force symmetrically and evenly on both sides to avoid damaging the threads or misaligning the bearing.

Distinguishing Bearing Types:

For deep groove ball bearings (which cannot withstand axial forces), the axial clearance is primarily controlled by the shims between the bearing outer ring and the bearing cap.

For angular contact ball bearings or tapered roller bearings (used in pairs to withstand axial forces), the clearance is controlled by adjusting the preload of the adjusting nut.

Considering Medium Temperature: For water pumps conveying high-temperature media, a larger amount of thermal expansion needs to be considered, and the operating axial movement should be appropriately increased.

Related Effects: Adjusting the axial clearance may slightly affect the compression of the mechanical seal or packing seal; this needs to be checked and confirmed accordingly.

 

Adjusting the axial clearance of a water pump is a delicate task. The core process can be summarized as follows: measure the total axial clearance → determine the target working axial clearance value → precisely position the rotor using the adjusting mechanism (nut or shim) → tighten and verify. As long as you follow the standardized steps, use the appropriate tools, and remain patient and careful, you can successfully complete this adjustment and ensure the long-term stable operation of the water pump. If you are unsure about the specific procedures, it is recommended to consult a professional maintenance technician.