According to news reporting originating from Alexandria, Virginia, by VerticalNews journalists, a patent by the inventors Masuda, Yui (Shizuoka, JP); Yamasaki, Tatsuya (Shizuoka, JP); Muramatsu, Makoto (Shizuoka, JP), filed on October 2, 2012, was published online on August 30, 2016. The assignee for this patent, patent number 9429487, is NTN CORPORATION (Osaka, JP). Reporters obtained the following quote from the background information supplied by the inventors: "Many of today's vehicle brake systems are hydraulic brake systems including a brake disk and friction pads adapted to be pressed against the brake disk by hydraulic cylinders. But with the introduction of new brake control systems such as anti-lock brake systems (ABS), electric brake systems, which require no hydraulic circuits, are receiving attention these days. "Typically, electric brake systems have a linear motion actuator including a rotary shaft to which the rotation of an electric motor is transmitted, and a linear motion mechanism for converting the rotation of the rotary shaft to a linear motion of a linear motion member. The linear motion actuator applies an axial load to a friction pad to press the friction pad against the brake disk, thereby generating a braking force. In order to control the braking force to a desired magnitude, a load sensor unit is mounted in many of such electric brake systems for detecting the magnitude of the axial load applied to the object. For improved response of the electric brake system, it is preferable to use a load sensor unit which can detect a load applied with a minimum possible movement of its moving part or parts. "A load sensor unit for use in a linear motion actuator which satisfies this requirement is disclosed e.g. in the below-identified Patent document 1. This load sensor unit includes an opposed pair of annular presser plates, piezoelectric crystal elements disposed between the presser plates, an insulating plate electrically insulating the piezoelectric crystal elements from one of the opposed pair of presser plates, and a lead wire through which voltage generated by the piezoelectric crystal elements are taken out. When an axial load is applied to this load sensor unit, the piezoelectric crystal elements generate a voltage corresponding to the load applied. It is thus possible to detect the axial load applied by measuring the voltage generated. Since the presser plates are moved very little relative to each other due to deformation of the piezoelectric crystal elements, this sensor unit will never deteriorate response of an electric brake system if mounted in the electric brake system. "But since the load sensor unit disclosed in Patent document 1 is designed such that an axial load applied to the sensor unit directly acts on the piezoelectric crystal elements, if the axial load is an impulsive load or a shear load, one or more of the piezoelectric crystal elements may crack or chip. This load sensor unit is therefore not sufficiently durable. "Under these circumstances, the inventors of the present application attempted to develop an improved load sensor unit which is sufficiently durable, and is capable of detecting loads with a minimum displacement of the moving parts of the sensor unit. As a result, the inventors proposed a magnetic load sensor unit including a flange member configured to be deflected when the reaction force to an axial force applied by the linear motion actuator to an object is applied to the sensor unit, a magnetic target which generates magnetic fields, and a magnetic sensor arranged such that when the flange member is deflected by the reaction force, the position of the magnetic sensor relative to the flange member changes. "Since this magnetic load sensor unit is configured such that the flange member is deflected when the reaction force to an axial force applied by the linear motion actuator to the object is applied to the sensor unit, the relative position between the magnetic target and the magnetic sensor changes due to the deflection of the flange member, and the output signal of the magnetic sensor changes corresponding to the change in relative position, it is possible to detect the magnitude of the axial load based on the output signal of the magnetic sensor. Since this magnetic load sensor unit is configured to detect the above-described axial load based on a change in relative position between the magnetic target and the magnetic sensor, which are kept out of contact with each other, this sensor unit is less likely to malfunction when impulsive loads or shear loads are applied thereto, and is thus sufficiently durable." In addition to obtaining background information on this patent, VerticalNews editors also obtained the inventors' summary information for this patent: "Object of the Invention "Such a load sensor unit is ordinarily mounted in a linear motion actuator such that the reaction force applied to the object is received by the sensor unit through a thrust bearing. Typically, the thrust bearing comprises an axially opposed pair of bearing washers, a plurality of rolling elements disposed between the opposed surfaces of the bearing washers, and a spacer keeping the rolling elements spaced apart from each other. The inventors of the present application discovered that when the sensor unit is used with one of the bearing washers in contact with the axial end surface of the flange member, hysteresis errors could occur in the loads as detected by the sensor unit. "In particular, it was discovered that with the thrust bearing in contact with the axial end surface of the flange member, there could be a difference between the detected value of a first axial load applied from the linear motion actuator which is increasing (thus increasing the degree of deflection of the flange member) and the detected value of a second axial load applied from the linear motion actuator which is decreasing (thus reducing the degree of deflection of the flange member), even if the first and second axial loads are the same, due to frictional force generated between the bearing washer of the thrust bearing and the flange member when the flange member is deflected by the axial load. "Thus, the inventors of the present application discovered that it would be possible to improve the detection accuracy of this type of magnetic load sensor by reducing such hysteresis errors. "If this linear motion actuator is mounted in an electric brake system, the linear motion actuator preferably has as short an axial length as possible to minimize the axial length of the electric brake system, because with this arrangement, parts surrounding the electric brake system (such as a suspension) can be laid out more freely. "An object of the present invention is to provide a load sensor for use in a linear motion actuator which is less likely to suffer from hysteresis errors and which reduces the axial length of the linear motion actuator, when used in the linear motion actuator. "Means for Achieving the Object "In order to achieve this object, the present invention provides a magnetic load sensor unit, for use in a linear motion actuator, configured to detect the magnitude of an axial load applied to an object from the linear motion actuator, wherein the load sensor unit comprises a flange member configured to be deflected when a reaction force to the axial load is received through a thrust bearing, a magnetic target which generates magnetic fields, and a magnetic sensor arranged such that the position of the magnetic sensor relative to the magnetic target changes when the flange member is deflected, wherein the flange member has an axial end surface on which a raceway is formed with which rolling elements of the thrust bearing are in rolling contact. "With this arrangement, since the flange member and the thrust bearing are in rolling contact with each other, no frictional force is generated therebetween when an axial load is applied and the flange member is deflected. This minimizes hysteresis errors. Since one of the conventional two bearing washers of the thrust bearing is not necessary, it is possible to reduce the axial length of the linear motion actuator by the axial thickness of one bearing washer. "If the thrust bearing is a thrust ball bearing including balls as the rolling elements, a groove having a circular arc-shaped section is preferably formed on the axial end surface of the flange member as the raceway. "If the thrust bearing is a thrust roller bearing including cylindrical rollers or needle rollers as the rolling elements, the raceway is preferably in the form of a hardened flat surface formed on the axial end surface of the flange member by heat treatment. "If the thrust bearing is a thrust self-aligning roller bearing including spherical rollers as the rolling elements, the raceway is preferably in the form of a concave surface inclined relative to the direction of the axial load and having a circular arc-shaped section. "Preferably, the magnetic target comprises at least two permanent magnets each magnetized in a direction perpendicular to a relative movement direction in which the position of the magnetic sensor relative to the magnetic target changes, wherein the permanent magnets are arranged such that opposite magnetic poles of the permanent magnets are aligned in the relative movement direction, and wherein the magnetic sensor is located in the vicinity of the boundary between the opposite magnetic poles. "With this arrangement, the magnetic load sensor unit has a directivity such that the output signal of the magnetic sensor changes steeply and sharply when the relative position between the magnetic target and the magnetic sensor changes in the axial direction but does not change so steeply when this relative position changes in a direction other than the axial direction. Thus, the output signal of the magnetic sensor is less likely to be influenced by external vibrations, so that it is possible to detect the magnitude of the axial load applied by the linear motion actuator with stable accuracy. "The magnetic sensor may be in the form of a magnetic resistance element or a magnetic impedance element. But from an economical viewpoint, a Hall IC is preferable. A Hall IC is especially preferable for use in the electric brake applicant because heat-resistant Hall ICs are now commercially available. If neodymium magnets are used as the permanent magnets, the magnets generate strong magnetic fields while taking up little space, so that the resolution of the magnetic load sensor unit improves. "The present invention also provides a linear motion actuator comprising a rotary shaft to which the rotation of an electric motor is to be transmitted, a linear motion member, a linear motion mechanism for converting the rotation of the rotary shaft to an axial movement of the linear motion member, thereby applying an axial load to an object, and a reaction force receiving member which receives a reaction force that acts on the linear motion mechanism when the axial load is applied to the object, wherein the above-described magnetic load sensor unit is used as the reaction force receiving member. "Advantages of the Invention "According to the magnetic load sensor unit, for use in a linear motion actuator, of the present invention, since the flange member and the thrust bearing are in rolling contact with each other, frictional force is less likely to be generated between the flange member and the thrust bearing when the flange member is deflected. This minimizes hysteresis errors. Since one of the conventional two bearing washers of the thrust bearing is not necessary, it is possible to reduce the axial length of the linear motion actuator by the axial thickness of one bearing washer." For more information, see this patent: Masuda, Yui; Yamasaki, Tatsuya; Muramatsu, Makoto. Magnetic Load Sensor Unit for Use in a Linear Motion Actuator, and Linear Motion Actuator. U.S. Patent Number 9429487, filed October 2, 2012, and published online on August 30, 2016.