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- Backlash clearance
It should be 3 to 6% of the module (on new equipment).
The gear generally works only on one side (except for reversible). The backlash clearance should be higher than 0 to avoid contact on two sides simultaneously. In this case, the risks of tooth breakage are high. A high value is safe.
- Ball mill level
Ball mill level
It should be considered for:
- Respect the process and material flow
- Avoid damage to the bearings axial stops
- Anticipate possible bearing wear and disappearance of bearing clearances
- Finally avoid contact patterns in extremity on pinion / girth gear major incident generator
In terms of value, the level of equipment should not change over (see table of values):
- Ball mill in horizontal position : + / – 0,4 mm / meter
- Tube in inclined position : + / – 0,03°
- Bearing clearance
Their role on the bearings, to create an oil film to separate the bearing shaft with hydrodynamic lubrication. The lack of bearing clearance prevents formation of the oil film. Metal metal contacts are possible. A scraping of bearings in case of falling bearing clearances is recommended.
Bearing contact pattern
They must not exceed the following value:
- For holder bearings of mills, 1/5 of holding surface
- For the kilns, 1/6 of holding surface
- For the pinion bearings, 1/4 of holding surface
- Burrs (gear)
They are observed on the gears, often normal and present no risk even if they are not removed by grinding.
Flow is the natural result of a combination between the material hardness (more hardness is the low, more the plastic flow is favored) and the sliding direction observed on the gears.
This applies regardless of the type of gear (straight, helix, chevron).
- The direction of flow on material on pinion from the primitive to the head and towards the foot. The result is naturally a burr on head and root step
- Girth gear creep directions are reversed and head to the primitive. Nothing justifies a burr on head. However this situation is regularly observed on kilns and is due to excessive wear on the rollers (formation of root step). This excessive wear should be treated by a grinding burrs intervention but also by treatment of the problem (reprofiling Novexa, turning or change in the level of wear).
They are dangerous. They correspond to a surface degradation by rupture. They often come from excessive Hertz pressure either by a failure of contact pattern, or by abnormal operating conditions.
Their development is done under layer by combination with the incompressible nature of the lubricant. It is therefore essential to stop their spread by mechanical machining and setting (Novexa response).
- Color code ( gears )
Color code ( gears )
- Green: Very low wear, ensuring low vibration level. The operating conditions are correct.
- Orange: The deformation requires closer monitoring. Intervention justified if it approach a red code.
- Red: The wear is excessive. Intervention is necessary. Major risks for the entire kinematic chain.
- Black: The wear is critical, urgent intervention. In general, this condition has already caused damage in the kinematic chain (breaks on the bearings, footings, foundations, shafts, couplings, gearbox or engine).
- Conicity defects
This defect may be linked to a rollers squewing or non-compliant calibrations. After machining and re-cylindrical shape, an adjustment is always necessary. The cones cause:
- Unplanned axial stresses (risk level)
- Poor distribution of loads on the rollers (poor balance)
- Constraints pushed downstream / upstream on the opposite tire (=> risk of tire squews wear)
- Contact patterns in extremity
Contact patterns in extremity
They present a major risk of rupture of gears and tires. The Hertzian pressure initially planned to cover 100% of the width, then increases so inversely proportional to the support width. This defect becomes really critical when the range does not exceed 30% of the width (pressure multiplied by 4). The increase in pressure can cause:
- The appearance of surface damage (pitting, chipping, seizing)
- Cracks or even tooth breakage (with high risk if the piece gets stuck between pinion and girth gear)
- An asymmetric wear generating a irreversible flatness defect in case of setting (loss of contact pattern if setting modification)
- On the teeth, a temperature difference higher than 4 degrees requires an adjustment.
- Contacts retro flank (gear)
Contacts retro flank (gear)
They are the result of insufficient backlash or just marks at equipment’s stop phases (unbalance and reverse rotation). We therefore recommend a control of operating clearances.
In the case of a projection on the extremity only, we recommend:
- For the gears, a control of pinion bearings
- For the gearboxes, control of bearing clearance
The best way to get usable information is either the temperature control in operation or applying a blue trace (active flank, retro flank) to confirm the load distribution.
- Cracks (gear and tire)
Cracks (gear and tire)
They are comparable in terms of risk of breakages. However breakage may be stabilized when the crack has a good chance to spread if left untreated. On repairs, the following must be met:
- Do not amplify the stress or the size of the crack during the heating phases, refueling and fall in temperature
- Check the absence of distortion during the operation (at least control of steps and Kd measure)
- Finally, after welding, machine profiles with a maximum tolerance of 0.1 mm (about gears and tires) that any company can not meet (this must be guaranteed)
- Drive bearings
They impose strict rules regarding diameters. If several rollers are drive bearings, they must have the same diameter (same for tires). The coating stations are often in this case.
- Driving report
It informs us on the load transfer conditions from one tooth to another. Too low driving report related to the design or incorrect setting conditions favor:
- An increase of wear speed, by interference
- A rise of vibrations by poor load transfer. This is especially true in the case of setting up a new pinion on a worn girth gear
- Equilibration of a tube
Equilibration of a tube
It requires a number of prior actions. It is important not to confuse a balance tube and balance of each roller.
A correctly adjusted tube should respect the balance of each roller individually. The worst scenario is to have a tube between its two stops but with two rollers which act strongly to the upside and 2 squewed rollers strongly acting downstream.
- Excessive pressure on the stops
Excessive pressure on the stops
She is still favored by a bad adjustment. The pressure can severely affect the stop and the flank until the break. A complete alignment control is required because it is likely that the slopes of the tube and some rollers are incorrect.
They appear on the tires during the processing of a circle in a multi-flat polygon. During rotation, the higher the speed is, the higher the vibrations are high. This defect may come from a setting default or wear in the driving element (chain or gear).
The vibrations often impose the reduction of production capacity. A machining on movable axis handles facets.
- Flatness defect (gears and tires)
Flatness defect (gears and tires)
They are often the result of the combined wear to defects like out of true or out of round. They generate:
- Axial stresses and blockages promoting vibratory rise
- Material flow with the appearance of little waves on the surface (touch sensitive). These alter the distribution of Hertz pressures and may develop pittings and chipping.
- A possible increase in vibration level
On tires, a setting and a machining are required. On the gears, only the Novexa’s reprofiling treats this defect.
- Lubricant viscosity
At a minimum, two characteristics are essential for the selection of characteristics:
- The viscosity is defined as a function of speeds and temperatures. We recommend to retain, at the operating temperature of 700 cSt for ball mill and from 1000 to 1500 cSt for kilns. The viscosity fell by half every 10 degrees above 40 degrees.
- The extreme pressure additives are essential to the working area (primitive), involved in reducing the coefficient of friction (reduces plastic deformation) and avoid micro seizures causing scratches, broken away and seizures. Graphite has the advantage.
- Material hardness
It directly affects the wear speed and the appearance of surface damage. We can summarize that the hardness of the pinion / roller should be about 15 kg higher than that of the girth gear / tire (40 to 50HB). The number of pinion cycle being 6 to 10 times higher, it should be harder to cover the wear gap. Even with this difference, pinion always wears out faster because its specific sliding is often higher than that of the girth gear.
- Meshing interference
It is materialized by the appearance of small scratches and broken away on a tooth head line or tooth bottom. In some cases, by damage like pitting or chipping in tooth root for the gears. Several sources are possible:
- Profile machining defect in the case of new gear
- Replacing a worn gear with a new one without action on the girth gear
- Setting default (bottom tooth too high, for example)
The interference often causes a vibratory increase.
- Micro pittings
They are to be analyzed carefully. This damage often appears on the soaked ground cemented teeth in tooth root on the pinions. It announces a problem of cutting interference, poorly distributed pressure and risk of transformation in chipping. An intervention is preferable, a minimum work on the quality of the lubricant.
- Mill trunnion (shaft)
Mill trunnion (shaft)
Shafts must be lubricated properly and must observe a flatness of less than 0.05 mm whatever the width of the shaft. The good condition of the shaft avoids scratching necessary to adjust the flatness of the bearing. In the case of major failure, poor contact pattern can damage the bearings and shaft. Cracks can appear on the bearings, repair is possible.
- Orange peel aspect (gear)
Orange peel aspect (gear)
It is not dangerous but confirms a combined material anomaly lubrication. It is a sign of a gradual wear and we recommend monitoring the speed of wear and deformation profiles using Novexa’s control templates.
- Out of roundness
Out of roundness
This is the radial movement of the rotating element during rotation. This measurement can be made using comparison, lead or inductive tools. The maximum acceptable value is 0.1 * module.
Out of round changes the position of primitive operating on the pinion at each girth gear rotation, the pinion increases wear speed and the Hertzian pressure on pinion step (risk of cracking).
- Out of true
Out of true
This is the axial movement of the rotating element during rotation. There are several techniques of measurement (1 or 2 comparators). The value must not exceed 0.5 / 1000mm in diameter.
During a gear reprofiling Novexa, this value does not matter because we take contact patterns by adjustment with blue.
- Pitch gap at the junction (gear)
Pitch gap at the junction (gear)
It always causes a clicking at the passage of the pinions; this difference is due to a defect of clamping or improper calibration. 2 cases are possible:
- Negative pitch. The pitch between two teeth is smaller than the theoretical pitch of the girth gear.
- Positive pitch. The pitch between two teeth is not greater than the theoretical pitch of the girth gear. This may be due either to excessive hold at the assembly or incorrect tightening.
Treatment of this type of fault can be done on site without dismantling, by on-site machining Novexa.
They are not dangerous. Especially never grind such damage because these degradations are located on the surface only. Some deeper size pittings may optionally be clipped (greater than 3 mm size).
Grinding reduces the contact patterns, increases the remaining Hertz pressure, accelerates the aging process and increases costs over time.
The pittings appear with the number of cycles, conditions of lubrication, and hardness. Their cycle goes through phases of stabilization and development. It does not affect the lifetime of the short-term part.
- Profile deformation
It generates unanticipated operating constraints on gear with new profile. The involute shape is the only one to permit, for the industry gears, an operation without shock or vibration. The deformation causes:
- A variation in angular speed which results in a vibration pulse to vertical dominant
- A variation of the operating pressure angle which, combined with an excessive backlash in the bearings, increases vibration
- An appearance of interference (root step and bump on its head) operating producing a predominantly horizontal vibration
- A modification of the sliding bearing and parameters (increase in the wear rate)
- Remaining thickness
It provides essential information on the choice of maintenance. This is essential in the calculation of the new gear with the definition of shearing coefficients and compression calculation coefficient. It must be above 80% for a green code. Contact us for more information.
The risks of prolonged use are:
- Fatigue crack appearance in tooth root on active flank or retro flank
- Risk of rupture of tooth piece in case of contact patterns reduction
- Acceleration of processes of fatigue and occurrence of surface damage
- Roller slope defect
Roller slope defect
The slope of a roller should not differentiate more than 0.06 ° relative to the slope of the tube. Measuring the slopes can be achieved by cold or hot measurement.
- Rollers ovality
This defect has more disadvantages than everyone imagines. Taken with comparators, the measures generate the following remarks:
- Rollers : This defect causes a load change at every turn roller. It thus increases the stresses and weakens the bearings. It varies the position of the axis of the tube increases the risk of degradation.
- Tire : This defect is a cumulation of possible distortion: circularity tire, tire thickness, adjustment under tire, shell defects and tube setting. Novexa machining provides an immediate solution.
- Root step on pinion (gear)
Root step on pinion (gear)
It increases the hertz pressure in the meshing, can cause fatigue cracks and generates significant vibrations mainly in areas of out of round eccentric girth gear the most important. In cases where intervention is not possible, the decline of tenths of mm is the most effective emergency solution.
But this solution is not sustainable because it changes the driving report.
In case of profile deformation, a reprofiling is recommended.
- Scratches (gear)
They are only found in areas with high slip between pinion and girth gear (head and tooth root).
They remain an area of minor damage. This damage appears in conventional manner in the case of high pressure combined with a slight contamination in the oil, in cases of insufficient contact pattern causing an increase in the normal pressure, or even in case of punctual lubrication defect.
- Seizures (gear)
They appear in case of recurrent breakdown of the lubricating film.
When seizing factor is too high (hertz pressure times year) and exceeds the limit of the lubricant resistance used, the first metal to metal contacts appear.
The seizures can be localized (surface defect and scope), on slide areas only (lubrication default), or finally on the entire surface (seizing that generated an increase in temperature). In this case, a shutdown of the installation, cleaning or reprofiling are needed.
- Shaft clearances / bearings
Shaft clearances / bearings
They measure only on the pinion bearings. The value should be 0.1 mm per every 100mm shaft diameter. The limit is 3 times that value.In cases of insufficient clearance, the risks are related to the expansion observed start. The shaft may expand more than the bearings. The result is the complete destruction of the bearing.
In cases of excessive backlash, vibration which is based on the backlashes of the line tends to be more important. Excessive amplitude may also cause damage to the bearing but also in the kinematic chain.
- Shoulders (gear and tire )
Shoulders (gear and tire )
They are the natural result of wear. Very often, pinions / rollers are larger than the girth gear/ tire or pinion is off-center relative to the girth gear. They appear on the pinions and rollers. The shoulders generate:
- Chipping, risks of tooth breakage or cracks in extremity
- An inability to properly regulate the clearance operation if the situation requires.
- Axial stresses and blockages promoting vibratory rise
It is always preferable to remove the steps by grinding (gear) or machining (rollers).
- Streaks in the rolling direction
Streaks in the rolling direction
They are an uncommon defect. Streaks are blocking the tube axially and derive from lubrication default, hardening or setting. We recommend a machining of tracks bearing of rollers and tires.
- Tooth breakage
It has multiple origins. The analysis in each case is required. Of course, its width is the most important but also its influence on the conduct report. In case the risk of expansion is zero, we confirm that a pinion can operate with a break of 30% of the width with straight teeth and 50% helical (depends on the helix angle). A girth gear can accept similar breaks but a reloading operation is preferable. (Read more)
- Tooth root backlash
Tooth root backlash
It is validated with the calculation based on : Module * 0.25 or tooth height / 9
- Zero tooth root backlash generates a degraded operation, the pinion behaves like a roller!
- Excessive tooth root backlash reduces the contact ratio, increases vibrations and the emergence of surface damage
These controls need to be systematic in the assembling or gear reprofiling.
- Triangulation defects
They are very common after years of operation and successive settings with inadequate tools. The angle of the roller is defined relative to two criteria:
- it defines the horizontal force (structural design) and the vertical force (load on foundations)
- the driving requirements (1 or more drive rollers, for example)
In cases when the angles exceed 35 °, it is not uncommon to tilt the bases at 45 °.
- Vibration amplitude
This is expressed only with the backlash in line (especially at high speeds, when the rotating tubes to over 10 rotations / minute)
- Backlash in bearings -> value of the remains, alignments defects, bad tooth background setting
- Backlash in the soles -> tightening, cracks, anchoring
- Backlash in the foundations -> Deteriorated foundations favor vibrational amplitude.
A lack of vibration is not a guarantee of quality. If the profile of the main harness is deformed, energy and frequency exists but can not be expressed with no backlash or destruction. Once a part will deteriorate in the line, the vibration will increase. The generated costs (risk of production shutdown and maintenance costs) will be much higher than a preventive decision.
They can be very destructive when the amplitude increases. Generally, 95% of the harness on vibrations (rotating tubes, ball mill and kilns) are collected at the meshing frequency. It is necessary to have the frequency to target the possible intervention area. The measures to be complete must be made on bearings, footings and foundations (top and bottom). A Novexa’s on-site machining provides effective solutions
- Wear speed
This parameter is a good indicator to track:
- The analysis of the conditions of lubrication and adjustment (lubricant, greasing mode, pollution, state of the joints etc …)
- The planning of the maintenance program and expenditure to consider
The measurement should be done with the proper tools (learn more).
- White line on the pitch (gear)
White line on the pitch (gear)
It is quite normal. The direction of flow caused by the rolling on the pitch and slip around precisely mark the teeth. On the pinion, when the white line (which is the primitive operation and therefore the working area) is very visible, it means that the out of round is very low. This mark is therefore an advantage that confirms the quality settings and girth gear geometry.