Description Structure for Sealing Input shaft and Box Body of Reduction Box Technical Field of the Invention The invention relates to a structure for sealing an input shaft and a box body of a reduction box. Background of the Invention In the prior art, in order to avoid leakage of lubricant in the box body of a reduction box, an O-ring is generally used for sealing an input shaft and a box body of the reduction box. However, for some application occasions with high requirements on sealing, for example, a motor for an electric wheelchair, lubricant leakage to a certain extent is still likely to occur with the above seal structure, resulting in inconvenience for users. Summary of the invention The purpose of the invention is to provide a structure for sealing an input shaft and a box body of a reduction box, which has good seal effect and is suitable for occasions with high requirement on the sealing level. The above technical purpose of the invention is mainly achieved by the technical solution as follows: a structure for sealing an input shaft and a box body of a reduction box is provided, comprising an input shaft and a box body; the input shaft is sleeved with a bearing, a shaft sleeve is provided between an outer ring of the bearing and the box body, and an O-ring is provided between the shaft sleeve and the box body; and a shaft shoulder is arranged on a position of the input shaft close to an inner end of the shaft sleeve, a framework oil seal is assembled on the shaft shoulder, and the framework oil seal is in liquid seal with the inner end of the shaft sleeve and the shaft 1 shoulder, respectively. Wherein, an outer wall of the shaft sleeve is sealed with the box body, and the shaft sleeve and the input shaft are sealed through the framework oil seal, thereby realizing the sealing between the input shaft and the box body. Therefore, the sealing performance of the reduction box is improved, so that the reduction box may be applied in occasions with high requirement on sealing. Preferably, a radial bulge loop extending in a radial direction is formed on a position of the shaft sleeve corresponding to the shaft shoulder of the input shaft, an inner diameter of the bulge loop is larger than a diameter of the shaft shoulder, an axial bulge loop extending in an axial direction is arranged on an inner end of the shaft sleeve, and an inner diameter of the axial bulge loop is between the inner diameter of the radial bulge loop and an outer diameter of the outer ring of the bearing; and, an inner wall of the framework oil seal is fitted with the shaft shoulder, the inner wall of the framework oil seal extends in the axial direction between the radial bulge loop of the shaft sleeve and the shaft shoulder and forms between the bearing and the shaft shoulder a circular portion fitted with an end face of the shaft shoulder, a first lip and a second lip are in turn formed on the framework oil seal from inside to outside, the first lip is fitted with an end face of the radial bulge loop of the shaft sleeve, and the second lip is fitted with an outer circumferential wall of the radial bulge loop of the shaft sleeve. Therefore, the invention is characterized in good seal effect and suitable for occasions with high requirement on the sealing level. Brief Description of the Drawings Fig. 1 is a structure diagram of the invention; Fig. 2 is an assembly structure diagram of an input shaft with a shaft sleeve and a framework oil seal in the invention; and, Fig. 3 is a structure diagram of the framework oil seal of the invention. 2 Detailed Description of the Invention The technical solutions of the invention will be further described in details as below by embodiments with reference to drawings. Embodiment 1 As shown in Fig. 1, Fig. 2 and Fig. 3, a flanging portion 14 is arranged on an outer end of a shaft sleeve 4, a stepped hole 15 fitted with an outer wall of the shaft sleeve 4 and the flanging portion 14 is arranged on the box body 3, the shaft sleeve 4 and the stepped hole 15 are assembled on the box body 3 in a fitted manner, and an O-ring 5 is provided between the shaft sleeve 4 and the box body 3. An input shaft 1 passing through the shaft sleeve 4 is provided with a bearing 2, an outer ring of the bearing is in close fit with an inner wall of the shaft sleeve 4, a shaft circlip 16 is arranged on the input shaft 1, and an inner circumference and an outer circumference of the shaft circlip 16 are clamped on the input shaft 1 and an inner wall of the shaft sleeve 6, respectively, for limiting the axial movement of the bearing 2. A shaft shoulder 6 is arranged on a position of the input shaft 1 close to an inner end of the shaft sleeve 4, a framework oil seal 7 is assembled on the shaft shoulder 6, and the framework oil seal 7 is in liquid seal with an inner end of the shaft sleeve 4 and the shaft shoulder 6, respectively. A radial bulge loop 8 extending in a radial direction is formed on a position of the shaft sleeve 4 corresponding to the shaft shoulder 6 of the input shaft 1, the radial bulge loop 8 and the shaft circlip 16 are fitted to limit the axial movement of the bearing 2, an inner diameter of the bulge loop 8 is larger than a diameter of the shaft shoulder 6, an axial bulge loop 9 extending in an axial direction is arranged on the inner end of the shaft sleeve 4, an inner diameter of the axial bulge loop 9 is between the inner diameter of the radial bulge loop 8 and an outer diameter of the outer ring of the bearing 2. An inner wall of the framework oil seal 7 is fitted with the shaft shoulder 6, the inner wall of the framework oil seal 7 extends in the axial direction between the 3 radial bulge loop 8 of the shaft sleeve 4 and the shaft shoulder 6 and forms between the bearing 2 and the shaft shoulder 6 a circular portion 10 fitted with an end face of the shaft shoulder 6. A first lip 11 and a second lip 12 are in turn formed on the framework oil seal 7 from inside to outside, a spiral spring 13 is provided between the first lip 11 and the second lip 12, the first lip 11 is fitted with an end face of the radial bulge loop 8 of the shaft sleeve 4, and the second lip 12 is fitted with an outer circumferential wall of the radial bulge loop 9 of the shaft sleeve 4. 4