JP6966170B2 - Diagnostic method of screw feed mechanism - Google Patents

Description

The present invention relates to a method of diagnosing pretension of a feed screw shaft in a screw feed mechanism provided in a machine tool or the like for linearly moving a moving body.

The machine tool is provided with a screw feed mechanism that feeds a moving body such as a table by rotating a feed screw shaft such as a ball screw. The feed screw shaft thermally expands due to heat generated by the nut portion and the bearing that supports the feed screw shaft, causing a positioning error in the moving body. Therefore, as disclosed in Patent Document 1, a method of applying a pretension to a feed screw shaft to suppress thermal expansion of the feed screw shaft and suppress a positioning error has been widely used.

In the screw feed mechanism, there are generally two methods for supporting the feed screw shaft, a structure in which the feed screw shaft is restrained in the axial direction by bearings at both ends (hereinafter referred to as "double anchor method"), and only one bearing. A restraining structure (hereinafter referred to as "single anchor method") is known.
In the former case, even if the feed screw shaft expands beyond the amount of deformation due to pretension, it can be restrained in the compression direction, so that thermal displacement is continuously suppressed and the axial rigidity of the feed screw shaft is also secured. It does not reduce the positioning accuracy. However, if the amount of thermal expansion of the feed screw shaft becomes large and the allowable load of the bearing is exceeded, it may be damaged.
In the latter case, the bearing is not damaged due to thermal expansion of the lead screw shaft, but the effect of suppressing thermal displacement is lost outside the range of the pretension action, and the axial rigidity is reduced, so that the positioning accuracy is lowered. In particular, in the single anchor method, it is important to control the amount of pretension because the performance deteriorates significantly as described above outside the pretension action range.
Therefore, conventionally, the thermal displacement at an arbitrary shaft position when a certain shaft operation is performed is measured, and the excess or deficiency of the pretension amount is empirically determined from the magnitude.

Jikkenhei 4-92743 Gazette

In the single anchor type screw feed mechanism, the conventional management method that measures the thermal displacement and empirically determines the excess or deficiency of the pretension amount is based on the difference in the amount of heat generated due to the individual difference of the bearing and the feed screw shaft and the operation history. Since the thermal displacements are different, it was not possible to accurately determine the excess or deficiency of the pretension amount.

Therefore, according to the present invention, in the single anchor type screw feed mechanism, it is possible to accurately determine the excess or deficiency of the pretension of the feed screw shaft, and the positioning accuracy is lowered and the bearing life is shortened due to the excess or deficiency of the pretension. The purpose is to provide a diagnostic method that can prevent the above.

In order to achieve the above object, in the invention according to claim 1, both ends of a lead screw shaft into which a moving body is screwed are rotatably supported by a support portion provided with a bearing, respectively, and the lead screw shaft is supported. A method of diagnosing pretension of a feed screw shaft in a screw feed mechanism in which a moving body can be screw-fed and moved by rotation and a bearing on one end side of the feed screw shaft is axially constrained in a support portion. And
The first temperature setting step of setting the temperature of the lead screw shaft to an arbitrary first temperature, and
The first actual movement distance acquisition step of rotating the feed screw shaft of the first temperature by a predetermined axis movement command to acquire the first actual movement distance of the moving body, and
A second temperature setting step that sets the temperature of the lead screw shaft to an arbitrary second temperature different from the first temperature, and
The second actual movement distance acquisition step of rotating the feed screw shaft of the second temperature by the same axis movement command as the first actual movement distance acquisition step to acquire the second actual movement distance of the moving body, and
The actual movement distance difference calculation step for calculating the difference between the first actual movement distance and the second actual movement distance,
A step of calculating the amount of thermal expansion of the lead screw shaft from the difference between the first temperature and the second temperature, and a step of calculating the amount of thermal expansion.
A thermal displacement suppression rate calculation step that calculates the thermal displacement suppression rate based on the difference in actual travel distance and the amount of thermal expansion,
It is characterized by performing a diagnostic step of diagnosing the pretension of the lead screw shaft from the obtained thermal displacement suppression rate.
According to the second aspect of the present invention, in the configuration of the first aspect, in the diagnostic step,
A support rigidity calculation step for calculating the support rigidity of the feed screw shaft from the thermal displacement suppression rate and the rigidity of the feed screw shaft set in advance, and
It is characterized by performing a support stiffness comparison step of comparing the support stiffness of the lead screw shaft with a preset threshold value.

According to the present invention, in the single anchor type screw feed mechanism, excess or deficiency of pretension of the feed screw shaft due to poor assembly or poor component accuracy, and reduction of pretension due to reduction of bearing preload due to bearing wear or the like can be achieved. It can be judged accurately. Therefore, the screw feed mechanism can be managed with high quality, and it is possible to prevent a decrease in positioning accuracy and a decrease in bearing life due to excess or deficiency of pretension.

It is explanatory drawing of the screw feed mechanism. It is explanatory drawing of the screw feed mechanism provided with the temperature setting means by an oil controller, and the actual movement distance acquisition means by a laser measuring machine. It is a flowchart of a diagnosis method. It is explanatory drawing of the screw feed mechanism using a probe sensor as an actual movement distance acquisition means. It is explanatory drawing of the screw feed mechanism using a linear scale as an actual movement distance acquisition means. It is a rigidity model figure in the axial direction of a lead screw shaft.

Hereinafter, embodiments of the present invention will be described with reference to the drawings.
FIG. 1 shows an example of a screw feed mechanism used in an NC machine tool. The screw feed mechanism 1 includes a feed screw shaft 2 such as a ball screw having a large-diameter screw portion 3 and small-diameter portions 4 and 4 at both ends thereof, and the small-diameter portions 4 and 4 of the lead screw shaft 2 are used as a base. It is rotatably supported by the support portions 6A and 6B installed on the base 5. A lead screw nut 7 is screwed into the threaded portion 3 of the lead screw shaft 2, a table 8 (FIG. 2) as a moving body is fixed to the lead screw nut 7, and a feed screw shaft is used by a servomotor (not shown). When 2 rotates and stops, the feed screw nut 7 is screwed in the axial direction so that the table 8 can move linearly and can be positioned at an arbitrary position. The position of the table 8 can be detected by a position detector (not shown).

The support portion 6A includes a bearing bracket 10 fixed on the base 5, and a sleeve 12 having a flange 13 is inserted from the outside into the stepped shaft insertion hole 11 formed in the bearing bracket 10. The sleeve 12 is positioned by the collar 14 provided between the flange 13 and the bearing bracket 10.
Three bearings 15, 15, ... Are arranged side by side in the sleeve 12 in the axial direction to pivotally support the small diameter portion 4 of the feed screw shaft 2. The inner ring of the bearing 15 is pressed from the outside to the end of the threaded portion 3 by the pressing nut 16 screwed into the small diameter portion 4 from the outside of the sleeve 12. On the other hand, the outer ring of the bearing 15 is pressed against the inner surface of the sleeve 12 by the lid 17 fitted in the outer opening of the sleeve 12. As a result, the bearings 15, 15 ... Are fixed in the axial direction in the sleeve 12 and are immovably supported. Therefore, here, by adjusting the width (thickness) of the collar 14 in the axial direction, pretension can be applied to the feed screw shaft 2.

On the other hand, also in the support portion 6B, the sleeve 12 with the flange 13 is inserted into the bearing bracket 10 from the outside, and the small diameter portion 4 of the feed screw shaft 2 is formed by the two bearings 18 and 18 arranged side by side in the sleeve 12. It is bearing. Here, too, the inner ring of the bearing 18 is pressed from the outside to the end of the screw shaft 3 by the pressing nut 16 screwed from the outside of the sleeve 12 to the small diameter portion 4, but the outer ring of the bearing 18 is a lid. Not restrained by etc. Therefore, the feed screw shaft 2 can expand in the axial direction on the support portion 6B side without receiving a reaction force in a state where there is no pretension.

Further, a through hole 20 is provided in the axial center of the feed screw shaft 2 over the entire length, and the through hole 20 is provided with a rotatable joint 21 and 21 and pipes 22 and 22 as shown in FIG. It is connected to the oil controller 23 via. Therefore, a medium (here, oil) whose temperature has been adjusted by the oil controller 23 can flow through the through hole 20 of the feed screw shaft 2. The oil controller 23 is connected to the NC device 24 so that the oil control temperature and the detection temperature can be exchanged with each other.
Further, a temperature sensor 25 is attached to the base 5, and temperature sensors 26 and 26 are attached to the bearing brackets 10 and 10, respectively, and connected to the temperature measuring unit 27. The bearing is a structure in the vicinity of the base 5 and the bearings 15 and 18. The temperature of the brackets 10 and 10 can be measured.

A method of diagnosing the pretension of the feed screw shaft 2 in the single anchor type screw feed mechanism 1 configured as described above will be described with reference to FIG.
First, in S1, the temperature of the lead screw shaft 2 is set to an arbitrary first temperature T1 (x) (first temperature setting step). The temperature setting means is illustrated below, but any method can be used as long as the temperature of the lead screw shaft 2 can be grasped.
For example, as shown in FIG. 2, the temperature adjusted by the oil controller 23 in the through hole 20 of the feed screw shaft 2 in the stationary state of the feed screw shaft 2 or in the shaft operation where it is determined that the heat generation is sufficiently small. The oil of Tc is flown, and this temperature Tc is set as the first temperature T1 (x) of the feed screw shaft 2.

Further, the temperature distribution in an arbitrary feed operation or a stationary state is estimated by a known method from the operation information obtained from the NC device 24 and / or the temperature information obtained from the temperature sensor 26, and the estimated temperature distribution is obtained. The representative temperature or the average temperature may be set as the first temperature T1 (x).
Further, the temperature distribution in an arbitrary feed operation or in a stationary state is directly measured by an optical fiber strain thermometer (not shown) installed inside the feed screw, and the representative temperature or average temperature of the measured temperature distribution is the first temperature. It may be T1 (x).
Then, the temperature distribution is obtained from the temperature information and the measurement position information measured by the direct temperature detecting means (not shown) such as the thermoelectric pair installed in the same manner, and the representative temperature and the average temperature of this temperature distribution are set to the first temperature T1. It may be (x).

Next, in S2, the NC device 24 outputs an axis movement command of an arbitrary distance to the servomotor, and acquires the first actual movement distance L1 with respect to the axis movement command distance L0 at the first temperature T1 (x) set in S1. (1st actual movement distance acquisition step). The actual travel distance acquisition means is illustrated below, but any method can be used as long as the actual travel distance can be acquired.
For example, as shown in FIG. 2, the first actual moving distance L1 may be acquired by measuring the moving distance of the target 31 placed on the table 8 using the laser measuring machine 30. The laser measuring machine 30 is connected to the NC device 24, and can exchange the measured distance information and the measurement operation with each other.

Further, as shown in FIG. 4, a probe sensor 37 which is a touch type and has an absolute length of L0'is attached to a spindle 36 rotatably supported by a spindle head 35 of a machine tool, while a known probe sensor 37 is mounted on a table 8. By installing a bar 38 having an absolute length and measuring the bar 38 with the probe sensor 37, the measurement distance L1'as the first actual movement distance may be acquired. At this time, the bar 38 may be installed anywhere including the base 5 as long as the dimension in the feed axis direction can be measured by the probe sensor 37. The probe sensor 37 is connected to the NC device 24 and can recognize the coordinates on the NC in contact with the bar 38. Further, the bar 38 is provided with a temperature sensor 39 and is connected to the temperature measuring unit 27 so that the temperature of the bar 38 can be detected.

Further, as shown in FIG. 5, in the scale feedback table 8 having the linear scale 40, the first actual movement distance L1 is measured from the difference in the detection positions of the linear scale 40 before and after the movement in the state where the scale feedback is disabled. You may. Here, the slider 41 of the linear scale 40 is supported by the slider support block 42 fixed to the base 5. The scale of the linear scale 40 is fixed to the table 8. The slider 41 and the NC device 24 are connected to each other, and the position of the table 8 can be directly detected. Further, the linear scale 40 is provided with a temperature sensor 43 and connected to the temperature measuring unit 27 so that the temperature of the linear scale 40 can be detected.

Next, in S3, the temperature of the lead screw shaft 2 is set to an arbitrary second temperature T2 (x) (second temperature setting step). The temperature setting means may be set by any method as long as the temperature distribution of the lead screw shaft 2 can be grasped.
For example, as in the setting of the first temperature, as shown in FIG. 2, in the stationary state of the lead screw shaft 2 or in the shaft operation where it is determined that the heat generation is sufficiently small, the feed screw shaft 2 is inserted into the through hole 20. Oil having a temperature Th adjusted by the oil controller 23 may be flown, and this temperature Th may be set to the second temperature T2 (x) = Th of the feed screw shaft 2.
Further, the temperature distribution in an arbitrary feed operation or a stationary state is estimated by a known method from the operation information obtained from the NC device 24 and / or the temperature information obtained from the temperature sensor 26, and the estimated temperature distribution is obtained. The representative temperature or the average temperature may be set as the second temperature T2 (x).
Further, the temperature distribution in an arbitrary feed operation or in a stationary state is directly measured by an optical fiber strain thermometer (not shown) installed inside the feed screw, and the representative temperature or average temperature of the measured temperature distribution is the second temperature. It may be T2 (x).
Then, the temperature distribution is obtained from the temperature information and the measurement position information measured by the direct temperature detecting means (not shown) such as the thermoelectric pair installed in the same manner, and the representative temperature and the average temperature of this temperature distribution are set to the second temperature T2. It may be (x).

However, the relationship between the following average feed screw temperature T1m at the first temperature T1 (x) and the following average feed screw temperature T2m at the second temperature T2 (x) must be T1m ≠ T2m.

Next, in S4, the NC device 24 outputs an axis movement command of the same distance as S2 to the servomotor, and sets the second actual movement distance L2 with respect to the axis movement command distance L0 at the second temperature T2 (x) set in S3. Acquire (second actual travel distance acquisition step). As with the first actual travel distance L1, the second actual travel distance L2 acquisition means also acquires the actual travel distance L2 using the laser measuring machine 30 or the linear scale 40, or uses the probe sensor 37 to acquire the second actual travel distance L2. It suffices to acquire the measured distance L2'as the actual moving distance, and the method does not matter.
Next, in S5, the difference ΔL between the first actual movement distance and the second actual movement distance is calculated (actual movement distance difference calculation step).
When L1 and L2 are acquired in S2 and S4, ΔL = L2-L1 is set, and when L1'and L2'are measured in S2 and S4, ΔL = L1'-L2'. .. Further, when the temperatures of the bar 38 and the linear scale 40 having a known absolute length are different when the first actual movement distance and the second actual movement distance are measured, the temperature (of the temperature sensor 39 and the temperature sensor 43) is different. The difference between these actual travel distances can be corrected by known means by using the detected value) and each linear expansion coefficient.

Next, in S6, the thermal expansion amount ε of the lead screw shaft 2 is calculated (thermal expansion amount calculation step). That is, the thermal expansion amount ε when the temperature of the lead screw shaft 2 whose movement in the axial direction is not suppressed changes from T1 (x) to T2 (x) is calculated by the following formula.

Next, in S7, the thermal displacement suppression rate η is calculated by the following formula (thermal displacement suppression rate calculation step).
η = ΔL / ε
Next, in S8, the support rigidity Ks of the feed screw shaft 2 is calculated by the following formula (support rigidity calculation step).

Next, in S9, the pretension of the lead screw shaft 2 is calculated (pretension calculation step). That is, when the axial rigidity model of the feed screw shaft 2 is represented by FIG. 6, the pretension p when the tension amount δ is represented by the following equation. In FIG. 6 and the following equation, Kf is the axial rigidity of the structure supporting the bearing 15, Ktacf is the axial rigidity of the bearing 15, Ktacr is the axial rigidity of the bearing 18, and Kr is the bearing 18. The axial rigidity of the structure that supports the bearing.

Next, in S10, the pretension and the threshold value are compared to make a diagnosis (pretension comparison step). That is, the pretension calculated in S9 is compared with an arbitrary threshold value set in advance in the NC device 24 to diagnose excess or deficiency of pretension (S8 to S10: diagnostic step). In this case, excess or deficiency may be diagnosed based on whether or not the pretension is within the range with a certain width of the threshold value, or a plurality of threshold values are set at equal intervals and the pretension is determined by which region the pretension belongs to. You may try to diagnose the excess or deficiency of.
Then, in S11, the diagnosis result of S10 is displayed on a monitor or the like (not shown) of the NC device 24.

As described above, according to the diagnostic method of the screw feed mechanism 1 of the above-described embodiment, the same actual movement distance is obtained between different temperatures, and the thermal expansion amount of the feed screw shaft is calculated from the temperature difference, and the actual movement distance difference and the thermal expansion are obtained. The thermal displacement suppression rate is calculated from the amount and the bearing rigidity is calculated from the thermal displacement suppression rate to obtain the pretension, and the obtained pretension is compared with the threshold value for diagnosis. In the screw feed mechanism 1 of the method, it is accurately determined that the pretension of the feed screw shaft 2 is excessive or insufficient due to poor assembly or poor component accuracy, and the decrease in pretension due to a decrease in bearing preload due to wear of the bearing 15 or the like. be able to. Therefore, the screw feed mechanism 1 can be managed with high quality, and it is possible to prevent a decrease in positioning accuracy and a decrease in bearing life due to excess or deficiency of pretension.

In the above embodiment, the pretension is diagnosed by comparing the calculated pretension with the preset threshold value, but the threshold value is not a constant but is set when the pretension is applied. It may be calculated by calculation using at least the tension amount δ0, the temperature distribution T0 (x) of the lead screw shaft 2, the temperature Tb0 of the base 5, the first temperature and the second temperature (threshold calculation step). Hereinafter, this calculation example will be described. Here, the coefficient of linear expansion α and the coefficient of linear expansion αb of the base 5 are also used.
First, the pretension p1 when the temperature distribution of the lead screw shaft 2 is T1 (x) and the temperature of the base 5 is Tb1 is calculated by the following formula.

Further, the pretension p2 when the temperature distribution of the lead screw shaft 2 is T2 (x) and the temperature of the base 5 is Tb2 is calculated by the following formula.

Using the above equation, for example, p1 may be set as the lower limit or its reference, and p2 may be set as the upper limit or its reference.
The base temperature is a representative temperature because the temperature distribution can be ignored, but the temperature distribution in the feed screw axis direction may be taken into consideration.

Then, here, the pretension and the threshold value are compared, and the calculated support rigidity Ks of the lead screw shaft 2 is compared with the preset threshold value (support rigidity comparison step) to diagnose the pretension. May be executed.
In addition, the specific structure of the screw feed mechanism, the temperature setting means, the actual moving distance acquisition means are not limited to the above-described form, and the number of bearings in the support portion, the pretension applying structure, and the like can be appropriately changed.

1 ... Screw feed mechanism, 2 ... Feed screw shaft, 3 ... Thread part, 4 ... Small diameter part, 5 ... Base, 6A, 6B ... Support part, 7 ... Feed screw nut, 8 ... Table 10, 10 ... Bearing bracket, 12 ... Sleeve, 14 ... Collar, 15, 18 ... Bearing, 16 ... Press nut, 17 ... Lid, 20 ... Through hole, 23 ... Oil controller, 24 ...・ NC device, 25, 26, 39, 43 ・ ・ Temperature sensor, 27 ・ ・ Temperature measuring unit, 30 ・ ・ Laser measuring machine, 37 ・ ・ Probe sensor, 40 ・ ・ Linear scale.

Claims (2)

Both ends of the feed screw shaft into which the moving body is screwed are rotatably supported by a support portion provided with a bearing, and the moving body can be screw-fed and moved by the rotation of the feed screw shaft, and the feed is described. A method of diagnosing the pretension of the feed screw shaft in a screw feed mechanism in which the bearing on one end side of the screw shaft is axially constrained in the support portion.
A first temperature setting step for setting the temperature of the feed screw shaft to an arbitrary first temperature, and
The first actual movement distance acquisition step of rotating the feed screw shaft at the first temperature according to a predetermined axis movement command to acquire the first actual movement distance of the moving body, and
A second temperature setting step of setting the temperature of the feed screw shaft to an arbitrary second temperature different from the first temperature, and
The second actual movement distance acquisition step of rotating the feed screw shaft at the second temperature by the same axis movement command as the first actual movement distance acquisition step to acquire the second actual movement distance of the moving body, and
The actual movement distance difference calculation step for calculating the difference between the first actual movement distance and the second actual movement distance, and
A step of calculating the amount of thermal expansion of the feed screw shaft from the difference between the first temperature and the second temperature, and a step of calculating the amount of thermal expansion.
A thermal displacement suppression rate calculation step for calculating the thermal displacement suppression rate based on the difference in the actual travel distance and the thermal expansion amount, and
A diagnostic step for diagnosing the pretension of the feed screw shaft based on the obtained thermal displacement suppression rate, and
A method of diagnosing a screw feed mechanism, which comprises performing. In the diagnostic step
A support rigidity calculation step for calculating the support rigidity of the feed screw shaft from the thermal displacement suppression rate and the preset rigidity of the feed screw shaft, and
The method for diagnosing a screw feed mechanism according to claim 1, wherein a support rigidity comparison step of comparing the support rigidity of the lead screw shaft with a preset threshold value is executed.

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