The spindle is the heart of the machine tool, and clues to 90% of shutdown failures can be found in the sound.
In a machining shop, the spindle of the machining center is the veritable "heart". It drives the cutting tool to rotate at thousands or even tens of thousands of revolutions per minute and withstands the repeated impact of cutting forces. Once there is a problem with the spindle, the quality of the machined surface will deteriorate and the dimensions will be out of tolerance, or the machine will be shut down directly, or even cause serious equipment accidents.
When many equipment maintenance personnel hear abnormal noises from the spindle, their first reaction is "Is the bearing broken? Take it apart and see." So I stopped the machine, disassembled it, and replaced the bearings. After two or three days of work, I put it back together and found that the abnormal noise was still there - the fundamental problem was not found at all.
Spindle fault diagnosis is not as simple as "taking it apart and looking at it". An excellent equipment maintenance engineer relies on systematic diagnostic thinking: first listen to the sound, then measure the vibration, then analyze the data, and finally pinpoint the source of the fault. Today we will start from actual combat and completely dismantle the diagnosis process of abnormal noise and vibration faults of the machining center spindle to help you find and solve the problem in the shortest time.
1. The first scene of abnormal spindle noise: first identify where the sound comes from
When the operator reports "there is abnormal noise in the spindle", the first step after the maintenance personnel arrive at the scene is not to disassemble the machine, but to "auscultate" it.
1. Timing of abnormal noise occurrence
Different sounding timings point to different fault locations. Maintenance personnel first need to ask the operator: Does the abnormal sound occur when the machine is turned on, or does it appear after a period of operation? Does it sound at low speeds or at high speeds? Is it making noise when cutting or idling?
If there is abnormal sound when the machine is turned on, it usually points to insufficient preload force of the bearing or poor lubrication; if there is abnormal sound after running for a period of time, it points to the gap change caused by thermal deformation; if it makes noise at low speed but not at high speed, it often means that the bearing rolling element is damaged; if it makes noise at high speed but not at low speed, it may be related to dynamic balance or bearing clearance.
If it makes a sound when cutting but does not make a sound when idling, it indicates that there is a problem with the tool or workpiece clamping, rather than a fault with the spindle itself; if it makes a sound when idling, it can basically be concluded that there is an abnormality within the spindle system.
2. The rhythm and timbre of abnormal noises
Spindle abnormal noise can be subdivided into several typical sounds, each sound corresponding to a different fault type:
Periodic "buzzing" sound: once every revolution, the rhythm is synchronized with the spindle speed. The common reason is that there is local damage to the rolling elements or raceways of the spindle bearing, such as pitting and spalling. This sound is usually more noticeable at low speeds.
Continuous "hissing" sound: high-frequency continuous noise, similar to the sound of metal friction. Points to poor lubrication of the bearing, dry grease or too small bearing clearance. If this kind of sound occurs after long-term high-speed operation, you should first suspect the lubrication system.
Dull "dong dong" sound: low-frequency impact sound with irregular rhythm. It may be that the spindle bearing cage is worn, the rolling elements fall off, or there is foreign matter inside the spindle. Once this sound occurs, stop the machine immediately for inspection, otherwise it will deteriorate rapidly.
Sharp "howling" sound: high-frequency sharp noise, usually occurs during high-speed operation. Common reasons are excessive grease in the main shaft bearing, or excessive bearing preload, leading to a vicious cycle of friction and heat generation. It may also be that the belt is slipping or the coupling is abnormal.
Irregular "clicking" sound: occasional crisp impact sound. Points to loose parts inside the spindle, abnormal broach mechanism, gearbox shifting mechanism failure, etc.
3. Consultation: Operator Communication Checklist Maintenance Personnel Must Master
Before starting the diagnosis, the maintenance personnel should ask the operator the following questions:
When did the spindle abnormal noise start? Did it appear suddenly or gradually worsen? Have the tools, tooling or procedures been replaced before and after the abnormal noise occurs? What is the spindle speed when abnormal noise occurs? How much is the feed? What materials are processed? Has the spindle reported an overload or temperature alarm? Has the spindle been serviced or repaired recently?
This information may seem trivial, but it can often directly identify the direction of the fault and avoid unnecessary disassembly and troubleshooting.
2. Quantitative detection of spindle vibration: data is more reliable than ears
The ear can tell the difference in sound but cannot quantify the severity of the vibration. After initially determining the direction of the fault, it is necessary to use instruments for quantitative detection to verify the auditory judgment.
1. Vibration meter detection
The portable vibration meter is the most basic tool for spindle fault diagnosis. Measure the effective value (mm/s) of the vibration speed of the spindle housing (near the front and rear bearings) and determine the vibration level according to the ISO 10816 standard:
Vibration speed ≤1.8mm/s: good condition
1.8～4.5mm/s: Acceptable status, but need to pay attention to the trend
4.5～11.2mm/s: Unqualified status, maintenance needs to be arranged
>11.2mm/s: Dangerous state, shut down immediately
Note: When measuring, the spindle should be idling and measured separately at different speed gears, and the changing trend should be recorded. If the vibration suddenly increases at a certain speed, it means that the resonance of the speed or the characteristic frequency of bearing failure is excited.
2. Vibration spectrum analysis
Portable vibration meters can only give overall vibration values, while spectrum analyzers can decompose vibration signals into different frequency components to locate specific fault sources. The following are the characteristic frequencies corresponding to common faults:
Bearing outer ring damage: The fault characteristic frequency is approximately 0.4 times the rotation frequency × the number of rolling elements.
Bearing inner ring damage: The fault characteristic frequency is approximately 0.6 times the rotation frequency × the number of rolling elements.
Rolling element damage: The fault characteristic frequency is approximately 0.23 times the rotation frequency × the number of rolling elements.
Cage damage: The fault characteristic frequency is about 0.4 times the rotation frequency.
The spindle is unbalanced: the vibration is mainly concentrated at 1 times the frequency.
Spindle misalignment: The main manifestation of vibration is an abnormal increase in amplitude at 2 times the frequency.
Gear meshing failure: vibration occurs at the gear meshing frequency and its multiples.
Through spectrum analysis, maintenance personnel can accurately determine which bearing and which component has the problem without blindly dismantling it.
3. Temperature detection
Infrared thermal imaging cameras or temperature patches can detect the temperature distribution on the surface of the spindle housing. The temperature at the front bearing of the spindle is obviously high (more than the ambient temperature 40℃), indicating that the bearing preload force is too large, the lubrication is poor, or the bearing has been severely worn.
The normal range of spindle temperature is generally controlled within the ambient temperature +30℃. If the temperature rises abnormally, even if there is no abnormal sound, a shutdown must be arranged for inspection.
4. Spindle accuracy detection
Use standard inspection rods and dial indicators to detect the radial runout and end face runout of the spindle taper hole:
Spindle taper hole radial runout (near the end face): precision grade ≤ 0.003mm, ordinary grade ≤ 0.008mm.
Spindle taper hole radial runout (at 300mm from the end face): precision grade ≤0.008mm, ordinary grade ≤0.015mm.
Spindle end face runout: precision grade ≤0.005mm, ordinary grade ≤0.010mm.
If the runout value is out of tolerance, it means that the spindle bearing is worn or the preload force is lost, and the bearing needs to be adjusted or replaced.
3. Five typical causes of spindle failure and their troubleshooting methods
Reason 1: Spindle bearing wear or damage
The bearing is the core component of the spindle and also the component with the highest failure rate. Long-term high-speed operation, cutting impact, and poor lubrication will cause pitting, spalling, and wear of bearing raceways and rolling elements.
Troubleshooting method: Replace new bearings. But you need to pay attention to the following points:
First, the bearing model must be consistent with the original one. It is best to purchase original or equivalent NSK, SKF, FAG and other brand products. Second, strictly control the preload force during installation. If the preload force is too small, the spindle will be insufficiently rigid and prone to vibration; if the preload force is too large, the bearing will heat up severely and its service life will be shortened sharply. Refer to the spindle manufacturer's maintenance manual and use a torque wrench or special tooling to tighten the locking nut to the specified torque. Third, the installation environment and tools must be clean.
Bearings are precision parts, and any tiny impurities mixed in may cause early failure. Fourth, run-in after installation is complete. The new bearing needs to be idle for 30 to 60 minutes at low speed to observe whether the temperature and vibration are normal, and then gradually increase the speed.
Reason 2: Poor spindle lubrication
There are two main ways of lubrication for spindle bearings: grease lubrication and oil-gas lubrication. For a grease-lubricated spindle, the grease will dry up and deteriorate after a certain period of time, resulting in dry friction of the bearings, high temperature and abnormal noise. For oil-air lubricated spindles, if the oil-air mixer is clogged, the pipeline leaks, or the compressed air pressure is insufficient, lubrication failure will also occur.
Troubleshooting:
If it is grease lubrication, replenish or replace the grease according to the specified period. When replacing, the old grease must be thoroughly cleaned, and new and old grease must not be mixed. If it is oil and gas lubrication, check the working status of the oil and gas mixer, whether the pipeline is smooth, and whether the compressed air pressure is within the range of 0.3~0.5MPa.
Reason 3: Poor spindle dynamic balance
When the cutting tool, broach mechanism, tool holder and other parts on the spindle rotate at high speed, if there is mass eccentricity, centrifugal force will be generated, causing the spindle to vibrate. The vibration increases abnormally after changing the tool, which is especially likely to occur in high-speed machining.
Troubleshooting method: Perform dynamic balance detection and correction on the spindle. Remove the spindle assembly, measure the unbalance and phase on the dynamic balancing machine, and then correct it by deweighting (grinding) or counterweighting. For machine tool spindles, dynamic balancing accuracy is generally required to reach G1.0 level or above. If dynamic balancing conditions are not met, at least the dynamic balancing levels of the tool holder and tool should be checked. The tool holder used for precision machining should be selected to be G2.5 or above, and a dynamic balance test should be performed after the tool is installed.
Reason 4: Failure of the spindle taper hole or broach mechanism
Poor coordination between the spindle taper hole and the tool holder taper surface, or the broach mechanism is worn, or the disc spring has insufficient elasticity, which will cause the tool to vibrate slightly during high-speed rotation, causing abnormal noise. This kind of failure often occurs after replacing the tool, and it is related to the way the operator installs the tool.
Troubleshooting method: Check whether there is any strain, rust, or wear on the surface of the spindle taper hole, and use a Morse taper gauge or a special taper detector to measure the contact rate. The taper hole contact rate should not be less than 80%. If the taper hole is damaged, it needs to be repaired. Check the broach mechanism, measure whether the broach force is up to standard, and whether the disc spring group is fatigued. For BT tool holders, the broaching force is generally between 8 and 12kN. If the broaching force is insufficient, the disc spring needs to be replaced or the broaching mechanism needs to be adjusted.
Reason 5: Spindle motor or transmission system failure
For belt-driven spindles, belt wear and slippage will cause speed fluctuations and abnormal noise. For direct-coupled spindles, wear and looseness of the coupling will produce periodic impact sounds. For electric spindles, damaged motor bearings and winding failures can also cause abnormal noise and vibration.
Troubleshooting method: Check the belt tension and press the middle of the belt with your fingers. The sinking amount should be between 10~20mm. Check the belt surface for cracks and wear. Check whether the coupling bolts are loose and whether the elastomer is aged. The failure of the electric spindle needs to be judged by professional testing equipment and sent back to the manufacturer for repair if necessary.
4. Standard process for spindle fault diagnosis: seven-step positioning method
In order to improve diagnosis efficiency, it is recommended to follow the following seven-step process to troubleshoot spindle faults:
The first step: stop the machine for consultation and collect fault information. Ask the operator about the time, speed, working conditions, and accompanying phenomena when abnormal noise occurs.
Step 2: Listen at idle to confirm the status of abnormal noise. Run idling at low speed, medium speed and high speed to determine the relationship between abnormal noise and speed.
Step 3: Measure vibration and temperature to quantify the degree of abnormality. Use a vibration meter and infrared thermal imaging camera to measure vibration and temperature at the spindle front bearing, rear bearing, motor base and other locations.
Step 4: Spectrum analysis to locate faulty components. Perform spectrum analysis on the vibration signal, compare the characteristic frequencies, and determine the location of bearing damage, imbalance or misalignment.
Step 5: Accuracy detection and quantify accuracy loss. Use inspection rods and dial indicators to detect the radial runout and end face runout of the spindle to determine the degree of accuracy deviation.
Step 6: Targeted disassembly and inspection to verify the diagnostic conclusion. Based on the conclusion of the above steps, dismantle and inspect relevant components in a targeted manner, and visually inspect the status of bearing raceways, cages, grease, etc.
Step 7: Repair or replace to restore spindle performance. Replace damaged parts, reassemble, adjust preload, run-in test, and finally restore the spindle accuracy and operating status.
5. Daily management of spindle maintenance: prevention is better than repair
The maintenance cost of spindle failure is extremely high - the price of one spindle may account for 20% to 30% of the price of the entire machine, and the downtime losses are huge. Rather than rush to repair after a failure, it is better to do routine maintenance.
1. Daily inspection
After starting the machine every day, let the spindle idle at low speed for 5~10 minutes to preheat, and then start processing after the temperature stabilizes. Preheating can effectively reduce thermal deformation and extend bearing life.
Operators should pay attention to the sound of the spindle during processing, and immediately stop the machine and report any abnormalities. After processing, clean the spindle taper hole and spindle end face, and apply anti-rust oil.
2. Weekly maintenance
Check the spindle lubrication system every week to confirm whether the lubricating oil level, oil pressure, and oil-air mixer are working properly. Check spindle belt tension. Clean the oil and dust on the spindle guard and heat sink.
3. Monthly Maintenance
Check the spindle vibration value every month and compare it with the baseline value to find the deterioration trend and arrange maintenance in advance. Check the spindle taper hole contact rate, clean and apply anti-rust oil. Check whether the broach mechanism moves smoothly and whether the broach force is normal.
4. Quarterly maintenance
Replace or replenish the spindle grease every quarter (according to the manufacturer's requirements), and clean the oil and gas lubrication system pipelines. Check the spindle cooling system, replace the coolant and clean the pipes.
5. Annual maintenance
A comprehensive spindle accuracy test is conducted every year, including radial runout, end face runout, temperature rise, vibration, noise and other items. It is recommended to contact the manufacturer or professional testing agency to complete. Determine whether bearing preload adjustment or overhaul is required based on the test results.
6. Review of typical cases
Case 1: Abnormal noise from the spindle of high-speed machining center
A machining center was processing aluminum alloy parts. The operator reported that the spindle made a "buzzing" sound when it rotated above 8000, and vibration patterns appeared on the machined surface.
Diagnosis process: Maintenance personnel conduct on-site listening to confirm that the abnormal noise is synchronized with the rotational speed. The vibration meter measures the vibration speed: 4.2mm/s at the front bearing and 2.8mm/s at the rear bearing, which are beyond the normal range. Spectral analysis shows an obvious peak at the characteristic frequency of the bearing outer ring.
Treatment result: The front bearing was disassembled and inspected, and it was found that there was obvious pitting corrosion in the outer ring raceway. After replacing the bearing of the same type and readjusting the preload force, the spindle vibration was reduced to 1.2mm/s, the abnormal noise was eliminated, and the vibration marks on the machined surface disappeared.
Case 2: The spindle makes abnormal noise when running at low speed but not at high speed.
On a certain turning and milling compound machine tool, the spindle makes a "clicking" sound when the spindle rotates below 500, but it is normal when running at high speed.
Diagnosis process: The rhythm of abnormal noise at low speed is synchronized with the speed. Use a stethoscope to assist in judging that the sound mainly comes from the front bearing of the spindle. Spectrum analysis found that there was a characteristic frequency peak of the cage in the low frequency band, and it was suspected that the cage was damaged.
Treatment results: After disassembly and inspection, it was found that the bearing cage had cracks and individual rolling elements were eccentrically worn. Troubleshooting after replacing bearings.
Case 3: Spindle vibration increases after replacing belt
After replacing the belt of a machine tool, the operator reported that the spindle vibration increased significantly and the surface quality deteriorated.
Diagnosis process: The vibration meter detects the vibration spectrum, and a large peak appears at 1 times the frequency, which is consistent with the unbalanced characteristics. Further inspection revealed that the pulley was installed eccentrically, resulting in an asymmetrical distribution of rotating variable mass.
Treatment result: Readjust the coaxiality of the pulley and lock the keyway connection. Check the vibration again and return to normal.
7. Conclusion
The spindle is the most precise and expensive core component of the machining center. Its health status directly determines the processing quality and machine tool life. By "listening to sound, measuring vibration, looking at temperature, checking accuracy" and other means, a systematic spindle health monitoring system can be established to detect signs of failure in time before it occurs.
Good maintenance is an investment, not a cost. Spending an extra 5 minutes on daily inspections may prevent a major overhaul costing tens of thousands of yuan and several days of downtime. We hope this article helps your team develop a more systematic approach to spindle maintenance and fault diagnosis.
If you have encountered difficult spindle failures in production, please leave a message in the comment area and let us discuss it together.
Source: Machinery Manufacturing Design
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