Patent Description:
As a type of sanding and polishing tool, a sander is more and more popular. Functionally, the sander is mainly used for cleaning, dehairing, and polishing surfaces in machinery, architectural decoration, and furniture painting. Structurally, the sander generally includes a housing, an electric motor, a fan assembly, an eccentric element, a base plate assembly, and the like. The housing is used for accommodating the electric motor, the fan assembly, and the eccentric element. The base plate assembly is at least partially disposed outside the housing. The base plate assembly is used for connecting sandpaper. The eccentric element drives the base plate assembly to move about a motor axis of the electric motor so that the sandpaper sands and polishes a workpiece to be sanded. The fan assembly is used for generating a dust collection airflow and a heat dissipation airflow when the electric motor operates.

To reduce the vibration generated when the sander operates, the sander is often provided with a balancing structure for balancing the base plate assembly. When the balancing structure is provided, to facilitate installation and simplify the structure, the balancing structure is directly fixedly connected to or integrally formed with the eccentric element and/or the fan assembly. However, such an arrangement tends to make the centroid of the balancing structure higher, and then the distance between the centroid of the balancing structure and the centroid of the base plate assembly increases, increasing the torque between the centroids. An additional counterweight needs to be added to balance the torque and ensure the smooth operation of the sander, increasing the weight of the sander and affecting the hand feeling of operation of the sander.

<CIT> (describing the preamble of claim <NUM>) discloses a multifunctional sander that includes a casing, a motor mounted in the casing, a switch controlling the motor to rotate bidirectionally, a first driving shaft driven by the motor, an eccentric device coupled with the first driving shaft, a grinding plate coupled with the eccentric device and an oscillating frame connected with the casing elastically. The sander also includes a unilateral clutch between the eccentric device and the oscillating frame. When the motor rotating in the first direction, the clutch makes the oscillating frame couple to the grinding plate and carry out orbital type grinding. When the motor rotating in the second direction, the clutch makes the oscillating frame uncouple with the grinding plate and carry out random orbit grinding.

A sander according to the present invention is defined by the independent claim <NUM>.

Advantageous embodiments are described in the dependent claims.

The present invention provides a lightweight sander.

The present invention adopts the technical solutions described below. A sander inter alia includes a housing; an electric motor at least partially disposed in the housing and used for providing a power source, where the electric motor includes a motor shaft that rotates about a motor axis; a fan assembly including a fan connected to the electric motor; an eccentric element driven by the electric motor, where the eccentric element has a central axis separated from the motor axis; and a base plate assembly including a base plate and a support for supporting the eccentric element, where the support includes a first end surface and a second end surface opposite to each other, and the second end surface is closer to a lower surface than the first end surface. The sander further includes a counterweight connected to the eccentric element and driven by the electric motor to rotate about the central axis. The ratio of the distance from the centroid of the counterweight to the lower surface of the base plate to the distance from the first end surface of the support to the lower surface of the base plate is greater than or equal to <NUM> and less than or equal to <NUM>.

The counterweight includes a body connected to the eccentric element; and a centroid adjustment portion for adjusting the centroid of the counterweight; where the centroid adjustment portion is fixedly connected to or integrally formed with the body.

The base plate assembly further includes a support base for mounting the support, where the support base is fixedly connected to the base plate and includes a bypass groove allowing the counterweight to rotate.

The base plate includes a groove allowing the counterweight to move, and the counterweight at least partially overlaps with the base plate in an axial direction.

The groove is a circular groove, the center of the groove is located on the central axis, and the ratio of the inner diameter of the groove to the outer diameter of the base plate is greater than or equal to <NUM> and less than or equal to <NUM>.

The counterweight includes a weight increasing portion and a weight reducing portion, where the weight reducing portion is disposed on a side of the counterweight facing the base plate, and the weight increasing portion is disposed on a side of the counterweight facing the fan.

The counterweight includes a weight increasing portion and a weight reducing portion, where the weight reducing portion is disposed on a side of the counterweight facing the fan, and the weight increasing portion is disposed on a side of the counterweight facing the base plate.

The product of the weight of the fan and the square of the outer diameter of the fan is greater than or equal to <NUM>·mm<NUM> and less than or equal to <NUM>·mm<NUM>.

The ratio of the weight of the fan to the mass of the electric motor is greater than or equal to <NUM>% and less than or equal to <NUM>%.

The sander further includes that the product of the weight of the fan and the square of the outer diameter of the fan is greater than or equal to <NUM>·mm<NUM> and less than or equal to <NUM>·mm<NUM>.

The fan is located on the upper side of the counterweight along the direction of the motor axis, where the density of the fan is less than <NUM>/cm<NUM>.

The groove is a circular groove coaxial with the central axis, and the ratio of the inner diameter of the groove to the outer diameter of the base plate is greater than or equal to <NUM> and less than or equal to <NUM>.

In the present invention, the centroid of the counterweight is adjacent to the lower surface of the base plate so that the torque between the centroid of the counterweight and the centroid of the base plate assembly is reduced, the weight of the counterweight is reduced, and the weight of the sander is reduced.

<FIG> shows a sander <NUM> that can drive a functional element to move, where the functional element may be sandpaper so that the sander <NUM> can sand and smooth surfaces of workpieces of various materials through the functional element.

It is to be understood that the sander <NUM> may specifically include round sandpaper, triangular sandpaper, square sandpaper, and so on. To clearly describe the technical solutions of the present invention, the round sander is used as an example in the present invention.

For ease of description, up, down, front, rear, left, and right as shown in <FIG> are defined.

As shown in <FIG>, the sander <NUM> includes a housing <NUM>, a switch <NUM>, a base plate assembly <NUM>, a power assembly <NUM>, a fan assembly <NUM>, an eccentric element <NUM>, and an energy source (not shown in the figure).

As the appearance of the sander <NUM>, the housing <NUM> is formed with at least a handle portion <NUM>, an accommodation portion <NUM>, and a bracket portion <NUM>. The handle portion <NUM> is used for a user to hold, where an end of the handle portion <NUM> is connected to the accommodation portion <NUM>, and the other end of the handle portion <NUM> may be used for connecting an external power cable or may form a connecting seat for mounting a portable direct current power supply such as a battery pack. The accommodation portion <NUM> is located between the handle portion <NUM> and the bracket portion <NUM>, an accommodation cavity is formed inside the accommodation portion <NUM>, and the power assembly <NUM> is at least partially disposed in the accommodation cavity. The bracket portion <NUM> is used for covering the fan assembly <NUM> and at least part of the base plate assembly <NUM>.

The switch <NUM> may be mounted on the housing <NUM>. Specifically, the switch <NUM> is mounted on the handle portion <NUM> so that it is relatively convenient to trigger the switch <NUM> when the user holds the handle portion <NUM>.

The power assembly <NUM> includes an electric motor <NUM>, where the electric motor <NUM> is used as a prime mover of the sander <NUM> and disposed in the housing <NUM>. The electric motor <NUM> includes a motor shaft <NUM> for transmitting power to the fan assembly <NUM>, and the motor shaft <NUM> rotates about a motor axis <NUM>. The motor axis <NUM> extends basically along an up and down direction.

The fan assembly <NUM> includes a fan <NUM> that can be driven by the motor shaft <NUM> to rotate about the motor axis <NUM>, and the fan <NUM> can generate an airflow when the fan <NUM> is driven by the motor shaft <NUM> to rotate.

The eccentric element <NUM> surrounds the motor shaft <NUM>, and the eccentric element <NUM> is eccentrically disposed relative to the motor shaft <NUM>. The eccentric element <NUM> is mounted to the motor shaft <NUM> and fixedly connected to the motor shaft <NUM>. It is to be noted that the eccentric element <NUM> being eccentrically disposed relative to the motor shaft <NUM> means that the eccentric element <NUM> has a central axis <NUM>, and the central axis <NUM> is parallel to the motor axis <NUM> of the motor shaft <NUM> and has a distance D from the motor axis <NUM> of the motor shaft <NUM>. The distance D exists so that when the motor shaft <NUM> rotates, the eccentric element <NUM> can transmit the rotation of the motor shaft <NUM> into the rotation and revolution of other components connected to the eccentric element <NUM>.

The motor shaft <NUM> drives the base plate assembly <NUM> so that the base plate assembly <NUM> can swing relative to the housing <NUM>. Specifically, the base plate assembly <NUM> is fixedly connected to the eccentric element <NUM>, that is to say, the motor shaft <NUM> transmits power to the base plate assembly <NUM> through the eccentric element <NUM>. The base plate assembly <NUM> includes a base plate <NUM>, the base plate <NUM> includes an upper surface <NUM> and a lower surface <NUM> opposite to each other, the lower surface <NUM> is disposed on a side of the base plate <NUM> facing away from the eccentric element <NUM> relative to the upper surface <NUM>, and the lower surface <NUM> is used for mounting the functional element such as the sandpaper. Driven by the motor shaft <NUM> and the eccentric element <NUM>, the base plate <NUM> can move eccentrically. When the base plate <NUM> moves eccentrically, the surface of a workpiece to be sanded can be continuously rubbed with the sandpaper, thereby implementing the function of sanding and polishing the workpiece to be sanded.

The energy source is used for providing a source of energy to the sander <NUM>. The energy source may be an alternating current or a direct current such as the battery pack or another portable mobile power supply. In this example, the energy source adopts the alternating current.

As shown in <FIG> and <FIG>, the base plate assembly <NUM> further includes a support <NUM> and a support base <NUM>. The support <NUM> is used for reducing a friction coefficient during the rotation of the motor shaft <NUM> and the eccentric element <NUM> disposed on the motor shaft <NUM>, thereby ensuring the rotational accuracy and parallelism of the motor shaft <NUM>. The support <NUM> is sleeved on the eccentric element <NUM> and fixedly connected to the eccentric element <NUM>. Specifically, the support <NUM> is a rolling bearing, where the rolling bearing <NUM> includes an outer race, an inner race, and balls. The inner race is sleeved on the motor shaft <NUM> and fixedly connected to the motor shaft <NUM>. The outer race is fixedly connected to the support base <NUM> and is disposed around the inner race. The balls are arranged between the outer race and the inner race and are used for enabling the outer race to move relative to the inner race.

The support base <NUM> is used for mounting the support <NUM>. The support base <NUM> is fixedly connected to the outer race of the support <NUM>, that is, the outer race of the support <NUM> is synchronously connected to the support base <NUM>. The support base <NUM> is fixedly connected to the base plate <NUM>, that is to say, the base plate <NUM> can move synchronously with the support base <NUM> and the outer race of the support <NUM>.

As shown in <FIG>, <FIG>, <FIG>, the sander <NUM> further includes a balancing assembly <NUM> for balancing the base plate assembly <NUM>. The balancing assembly <NUM> includes a counterweight <NUM> located between the fan <NUM> and the base plate <NUM> along the direction of the motor axis <NUM>, and the counterweight <NUM> is detachably and fixedly connected to the eccentric element <NUM>. That is to say, the eccentric element <NUM> moves synchronously with the counterweight <NUM> and the counterweight <NUM> can rotate with the eccentric element <NUM> about the central axis <NUM>. It is to be noted that the counterweight <NUM> being detachably and fixedly connected to the eccentric element <NUM> means that the counterweight <NUM> and the eccentric element <NUM> are two separable parts and can be fixed together through a screw connection, a threaded connection, or a snap connection to rotate synchronously. In the present application, the counterweight <NUM> and the eccentric element <NUM> are connected using a screw. Specifically, the balancing assembly <NUM> includes at least one fixing member, and the fixing member is specifically a positioning screw. The positioning screw penetrates through a connecting hole of the counterweight <NUM> so that the counterweight <NUM> abuts against the inner race of the support <NUM>, and finally, the counterweight <NUM> is locked to the eccentric element <NUM>, thereby achieving the synchronous rotation of the counterweight <NUM> and the eccentric element <NUM>. It may also be understood as that the fixing member penetrates through the connecting hole of the counterweight <NUM> from a side of the counterweight <NUM> facing the base plate <NUM> so that the counterweight <NUM> is fixed to the eccentric element <NUM>, that is, the counterweight <NUM> rotates synchronously with the eccentric element <NUM>. The counterweight <NUM> supports the inner race of the support <NUM>, that is to say, the inner race of the support <NUM> can be prevented from being disengaged from the outer race of the support <NUM> during the operation of the sander <NUM> without an additional positioning retainer ring, thereby simplifying the structure and making the structure of the whole machine more compact.

In this example, the support <NUM> includes a first end surface <NUM> and a second end surface <NUM> opposite to each other, where the second end surface <NUM> is disposed on a side of the support <NUM> facing the lower surface <NUM> of the base plate <NUM> relative to the first end surface <NUM>. That is to say, the first end surface <NUM> is disposed on the upper side of the second end surface <NUM> along the direction of the motor axis <NUM>. The ratio of the distance H1 from the centroid A of the counterweight <NUM> to the lower surface <NUM> of the base plate <NUM> to the distance H2 from the first end surface <NUM> of the support <NUM> to the lower surface <NUM> of the base plate <NUM> is greater than or equal to <NUM> and less than or equal to <NUM>. The centroid A of the counterweight <NUM> is adjacent to the base plate <NUM>, and the position of the centroid A of the counterweight <NUM> is set within the preceding range so that the centroid A of the counterweight <NUM> is adjacent to the centroid B of the base plate assembly <NUM>. In this manner, the distance between the centroid A of the counterweight <NUM> and the centroid B of the base plate assembly <NUM> can be reduced, thereby greatly reducing the weight of the counterweight <NUM> and reducing the weight of the entire sander <NUM>.

As shown in <FIG>, the centroid A of the counterweight <NUM> is located between the base plate <NUM> and the support <NUM>, where the distance D1 from the centroid A of the counterweight <NUM> to the centroid B of the base plate assembly <NUM> along the direction of the motor axis <NUM> is greater than or equal to <NUM> and less than or equal to <NUM>. The counterweight <NUM> is used for achieving a double balance of mass balance and torque balance of the base plate assembly <NUM>. Through the preceding arrangement, the centroid A of the counterweight <NUM> is adjacent to the centroid B of the base plate assembly <NUM>, the distance from the centroid A of the counterweight <NUM> to the centroid B of the base plate assembly <NUM> is reduced, and the torque between the centroids is reduced. That is, the counterweight <NUM> can reduce the weight for balancing torque. It may also be understood as that no additional weight is required to balance the torque so that the following case can be avoided: the weight of the counterweight <NUM> is added to balance other weights. In other words, the torque can be counteracted simply through the configuration of other very light weights, and thus only an additional weight having the same weight as the other weights needs to be configured on the counterweight <NUM>. Therefore, it can be seen that through the preceding arrangement, the weight of the counterweight <NUM> can be greatly reduced, thereby reducing the weight of the sander <NUM>, facilitating user operation, reducing the weight of the whole machine, and reducing the fatigue of the user. The distance D1 from the centroid A of the counterweight <NUM> to the centroid B of the base plate assembly <NUM> is greater than or equal to <NUM> and less than or equal to <NUM>. The distance D1 within the preceding range ensures a better effect.

The counterweight <NUM> includes a body <NUM> and a centroid adjustment portion <NUM>. The body <NUM> is connected to the eccentric element. The centroid adjustment portion <NUM> is used for adjusting the centroid of the counterweight <NUM>, and the centroid adjustment portion <NUM> is fixedly connected to or integrally formed with the body <NUM>. In this example, the centroid adjustment portion <NUM> is integrally formed with the body <NUM>. Specifically, the centroid adjustment portion <NUM> includes a weight increasing portion and a weight reducing portion, where the weight increasing portion is used for increasing the weight of the counterweight <NUM>, and the weight reducing portion is used for reducing the weight of the counterweight <NUM>. The weight reducing portion is disposed on a side of the counterweight <NUM> facing the base plate <NUM>, and the weight increasing portion is disposed on a side of the counterweight <NUM> facing the fan <NUM>. Of course, as another example, the weight reducing portion is disposed on the side of the counterweight <NUM> facing the fan <NUM>, and the weight increasing portion is disposed on the side of the counterweight <NUM> facing the base plate <NUM>. Through the preceding arrangement, that is, the centroid adjustment portion <NUM> is disposed on the counterweight <NUM>, the weight increasing portion and the weight reducing portion may be adjusted according to the weight and eccentricity of the base plate assembly <NUM>, so as to adjust the position of the centroid and achieve the mass balance and torque balance of the base plate assembly <NUM>. For example, the centroid adjustment portion <NUM> extending upward for increasing the weight of the counterweight <NUM> is provided, or the centroid adjustment portion <NUM> extending downward for reducing the weight of the counterweight <NUM> is provided.

As shown in <FIG>, the counterweight <NUM> is located between the support base <NUM> and the base plate <NUM>, and the counterweight <NUM> is at least partially disposed within the base plate <NUM>. The counterweight <NUM> includes a first surface <NUM> and a second surface <NUM> opposite to each other, where the first surface <NUM> faces away from the base plate <NUM> relative to the second surface <NUM>, and the second surface <NUM> is located on the lower side of the fixing member along the direction of the motor axis <NUM>. That is to say, the first surface <NUM> at least partially abuts against the inner race of the support <NUM>, and the second surface <NUM> is adjacent to the upper surface <NUM> of the base plate <NUM>. The upper surface <NUM> of the base plate <NUM> is recessed downward to form a groove <NUM> in which the counterweight <NUM> is movable, and the second surface <NUM> of the counterweight <NUM> extends toward a direction in which the groove <NUM> is recessed, that is to say, the counterweight <NUM> partially overlaps with the base plate <NUM> in an axial direction. In this example, the groove <NUM> is a circular groove coaxial with the central axis <NUM>, and the ratio of the inner diameter of the groove <NUM> to the outer diameter of the base plate <NUM> is greater than or equal to <NUM> and less than or equal to <NUM>. The radius C of the groove <NUM> is greater than or equal to <NUM> and less than or equal to <NUM>. On the one hand, the radius C of the groove <NUM> is set within the preceding reasonable range so that it is ensured that the counterweight <NUM> has enough space for movement, and the following case is avoided: the radius of the groove <NUM> is so large that a dimension of a transmission assembly in a radial direction is increased. On the other hand, the groove <NUM> recessed downward is provided on the base plate <NUM> and the counterweight <NUM> is disposed as close to the base plate <NUM> as possible so that the centroid A of the counterweight <NUM> is closer to the base plate <NUM>, and the dimension of the whole machine in the up and down direction can be reduced while the torque is reduced, thereby making the structure of the whole machine more compact and reducing the weight of the whole machine.

The counterweight <NUM> is at least partially disposed in the support base <NUM>, and the support base <NUM> is formed with a bypass groove <NUM> and an accommodation groove <NUM>. The bypass groove <NUM> is located on the outer side of the accommodation groove <NUM> in a radial direction perpendicular to the motor axis <NUM>, and an end surface of the support base <NUM> facing the upper surface <NUM> of the base plate <NUM> is recessed upward to form the bypass groove <NUM> and the accommodation groove <NUM>. The accommodation groove <NUM> is used for accommodating the support <NUM>, where a wall surface of the accommodation groove <NUM> is at least partially in contact with the outer race of the support <NUM>. The bypass groove <NUM> partially overlaps with the support <NUM> in the axial direction. The bypass groove <NUM> is used for the counterweight <NUM> to move, where the radius E of the bypass groove <NUM> is greater than or equal to <NUM>. In this manner, the structure of the whole machine can be more compact in the up and down direction while it is ensured that the counterweight <NUM> has enough space for movement, thereby providing the user with a compact and lightweight sander <NUM>.

The eccentric element <NUM> is fixedly connected to or integrally formed with the fan <NUM>. In this example, the eccentric element <NUM> is integrally formed with the fan <NUM>, that is, the fan <NUM> is formed with the eccentric element <NUM>. The fan <NUM> is mounted on the motor shaft <NUM> and can be driven by the motor shaft <NUM> to rotate. Of course, in other words, the fan <NUM> is the eccentric element <NUM>.

The ratio of the weight of the fan <NUM> to the weight of the electric motor <NUM> is greater than or equal to <NUM>% and less than or equal to <NUM>%. In the case where the weight of the electric motor <NUM> is constant, the smaller ratio of the weight of the fan <NUM> to the weight of the electric motor <NUM> means that the fan <NUM> is lighter and a smaller moment of inertia is generated when the fan <NUM> rotates, thereby greatly improving the feeling of operation of the user. On the other hand, when the fan <NUM> is lighter, the entire sander <NUM> is lighter so that the sander <NUM> is convenient for the user to hold.

In this example, the fan <NUM> is made of a material with a density less than <NUM>/cm<NUM>. Since the weight is proportional to the density, that is, the smaller the density, the smaller the weight of the fan <NUM>. That is, the weight of the fan <NUM> is effectively reduced, thereby reducing the weight of the sander <NUM>. For example, when the fan <NUM> is made of a material with a density greater than or equal to <NUM>/cm<NUM> and less than or equal to <NUM>/cm<NUM>, the weight of the fan <NUM> can be reduced while structural strength is satisfied. The fan <NUM> may be made of aluminum so that costs are saved while the weight of the fan <NUM> is reduced. When the sander <NUM> is placed as shown in <FIG> and the electric motor <NUM> drives the fan <NUM> to rotate, the fan <NUM> generates a moment of inertia, that is to say, when the fan <NUM> is rotated, a force of constraint is formed to keep the fan <NUM> rotating about the motor axis <NUM> (that is, the up and down direction). The force of constraint restricts the sander <NUM> to rotating around the up and down direction when the sander <NUM> has a tendency to move in a certain direction obliquely intersecting with the up and down direction. That is to say, when the user applies a force to the sander <NUM> to move the sander <NUM> in a certain direction obliquely intersecting with the up and down direction, the force of constraint causes the sander <NUM> to have a tendency to move in a direction opposite to the movement direction of the sander <NUM>. Thus, the user is required to apply a greater force to overcome the force of constraint, causing inconvenience of operation. The user operating in such a manner for a long time easily feels fatigued, affecting working efficiency. The moment of inertia is proportional to the weight. The greater the weight of the fan <NUM>, the greater the moment of inertia and the greater the effect on the user. Therefore, through the preceding arrangement, the moment of inertia can be reduced, thereby improving user experience.

The product of the weight of the fan <NUM> and the square of the outer diameter of the fan <NUM> is greater than or equal to <NUM>·mm<NUM> and less than or equal to <NUM>·mm<NUM>. The outer diameter refers to the diameter of the outer edge of the fan <NUM>. The product of the weight of the fan <NUM> and the square of the outer diameter of the fan <NUM> is set within the preceding range so that the moment of inertia generated when the fan <NUM> rotates can be effectively reduced, the effect of the force of constraint during the operation of the user is reduced, and thus the working efficiency is improved.

The total weight of the fan <NUM> and the electric motor <NUM> is less than or equal to <NUM>. In the internal structure of the sander <NUM>, the electric motor <NUM> and the fan <NUM> are much heavier than other components, that is to say, the electric motor <NUM> and the fan <NUM> mainly contribute to the weight of the sander <NUM>. The structures and positions of the balancing assembly <NUM> and the fan assembly <NUM> are configured so that the weight of the fan <NUM> is greatly reduced, thereby obtaining the sander <NUM> with a relatively small moment of inertia and a relatively small weight. In this example, the weight of the fan <NUM> is less than or equal to <NUM>.

As shown in <FIG>, an air inlet, an air outlet, and a dust outlet are formed on the housing <NUM>. The fan <NUM> is fixedly connected to the motor shaft <NUM>, that is, the fan <NUM> operates as the electric motor <NUM> rotates. When the fan <NUM> rotates, the airflow enters from the air inlet, flows through the electric motor <NUM> and other components, and finally flows out from the air outlet, so as to achieve the effect of heat dissipation on the electric motor <NUM> and other components in the housing <NUM>. On the other hand, the airflow can effectively blow dust on the base plate <NUM> to the dust outlet, and the dust finally enters a dust collection box (not shown in the figure), thereby achieving a dust collection effect.

Claim 1:
A sander (<NUM>), comprising:
a housing (<NUM>);
an electric motor (<NUM>) at least partially disposed in the housing and used for providing a power source, wherein the electric motor comprises a motor shaft (<NUM>) that rotates about a motor axis (<NUM>);
a fan assembly (<NUM>) comprising a fan (<NUM>) connected to the electric motor;
an eccentric element (<NUM>) driven by the electric motor, wherein the eccentric element has a central axis (<NUM>) separated from the motor axis; and
a base plate assembly (<NUM>) comprising a base plate (<NUM>), a support (<NUM>) for supporting the eccentric element, and a support base (<NUM>), wherein the support base is used for mounting the support, wherein the support base is fixedly connected to an outer race of the support and to the base plate, wherein the base plate includes an upper surface (<NUM>) and a lower surface (<NUM>) opposite to each other, wherein the lower surface is disposed on a side of the base plate facing away from the eccentric element relative to the upper surface, and the lower surface is used for mounting the functional element such as a sandpaper, wherein the support comprises a first end surface (<NUM>) and a second end surface (<NUM>) opposite to each other, and the second end surface is closer to the lower surface than the first end surface;
wherein the motor shaft can drive the base plate assembly so that the base plate assembly can swing relative to the housing, wherein the base plate assembly is fixedly connected to the eccentric element, wherein the eccentric element surrounds the motor shaft;
characterized in that,
the sander further comprises a counterweight (<NUM>) connected to the eccentric element and driven by the electric motor to rotate about the central axis;
wherein a ratio of a distance from a centroid of the counterweight to the lower surface of the base plate to a distance from the first end surface of the support to the lower surface of the base plate is greater than or equal to <NUM> and less than or equal to <NUM>; and
wherein between the electric motor and the base plate assembly, starting from the electric motor and going to the base plate assembly: the electric motor, the fan assembly, the eccentric element, the support plate, the support, the counterweight, and then the base plate are arranged, wherein at least the fan is located in the housing.