Description Structure for Assembling Motor End covers and Housing Technical Field The invention relates to a motor assembly structure, in particular to a structure for assembling motor end covers and a housing, which is convenient for assembly and can prevent damaging a winding of a motor coil. Background of the Invention The existing structure for assembling motor end covers and a housing is commonly as follows: the front end cover and the rear end cover are fixed at two ends of the housing through bolts and nuts, wherein the bolts are fixedly connected with the front and the rear end covers at the two ends of the housing after penetrating the housing, the assembly process is complex, a winding of a coil in the housing is very easily damaged during the assembly process, the performances of a motor are affected, the product qualification rate is reduced, and the motor can be scrapped when the situation is serious. Summary of the Invention The invention aims at providing a structure for assembling motor end covers and a housing, which is simple in structure and convenient for assembly and can prevent damaging a winding of a motor coil, further improve the product qualification rate, and solve the technical problems that the assembly process of the motor end covers and the housing is time-consuming and labor-consuming, the winding of the coil in the housing is very easily damaged, the performances of a motor are further affected, the product qualification rate is reduced, the motor can be directly scrapped when the situation is serious and the like in the prior art. The above technical purpose of the invention is mainly solved through the following technical scheme: the structure for assembling the motor end covers and the housing comprises the front end cover and the rear end cover, as well as the housing which is coaxially clamped between the front and the rear end covers, front end cover lugs are arranged on the radial outer edge surface of the front end cover, rear end cover lugs corresponding to the front end cover lugs are arranged on the radial outer edge surface of the rear end cover, screw rods are connected between the front end cover lugs and the rear end cover lugs, and the screw rods are arranged outside the housing and used for clamping the front end cover and the rear end cover at two ends of the housing. By arranging the front and the rear end cover lugs on the radial outer edge surfaces of the 1 front and the rear end covers, the front and the rear end covers can be clamped at two ends of the housing by enabling the screw rods to penetrate the front and the rear end cover lugs, the structure is simple, the weight is light and the cost is low; and the screw rods are arranged on the outer sides of the housing, the assembly is convenient, the damages to the winding of the motor coil of the housing can be avoided, and the product qualification rate can be improved. Preferentially, three to four front end cover lugs are arranged on the radial outer edge surface of the front end cover, and the front end cover lugs are uniformly distributed along the circumferential direction of the front end cover. When the three to four front end cover lugs are uniformly distributed in the circumferential direction of the front end cover, the clamping force of the front and the rear end covers at the two ends of the housing can be ensured to be balanced and the deflection of the front and the rear end covers can be prevented. Preferentially, the front end cover lugs and the rear end cover lugs are planar bosses with semi-circular cross sections, the front end cover lugs are integrally connected with the front end cover, the inner end surfaces of the front end cover lugs are flush with the inner end surface of the front end cover, the rear end cover lugs are integrally connected with the rear end cover, and the inner end surfaces of the rear end cover lugs are flush with the inner end surface of the rear end cover. The front and the rear end cover lugs which are integrally molded on the front and the rear end covers are simple for processing, low in cost and high in strength; when the inner end surfaces of the front and the rear end cover lugs are flush with the inner end surfaces of the front and the rear end covers, the clamping force of the front and the rear end covers can be improved, the length of the screw rods connected between the front and the rear end cover lugs is reduced, and the cost is saved. More preferentially, reinforcing ribs are arranged on the front end cover lugs and the rear end cover lugs, and the reinforcing ribs are arranged on two sides of the front end cover lugs and the rear end cover lugs along the axial direction. By arranging the axial reinforcing ribs at outer side ends of the front and the rear end cover lugs, the connection strength between the front and the rear end cover lugs and the front and the rear end covers can be improved, and the rupture of the front and the rear end cover lugs under the action of pulling force of the screw rods can be prevented. Therefore, the structure for assembling the motor end covers and the housing has the following advantages: the front and the rear end cover lugs are arranged on the radial outer edge surfaces of the front and the rear end covers for fixing the front and the rear end covers, the structure is simple, the weight is light and the cost is low; the screw rods are arranged on the outer sides of the housing, the assembly is convenient, the damages to the winding of the motor coil of the housing can be simultaneously avoided, and the product qualification rate can be improved; when the three to four front and the rear end cover lugs are uniformly distributed in the circumferential directions of the front and the rear end covers, the deflection of the front and the rear end covers can be prevented, and the assembly quality can be improved; by arranging the axial reinforcing ribs on the outer side ends of the front and the rear end cover 2 lugs, the connection strength of the front and the rear end cover lugs can be improved. Brief Description of the Drawings Figure 1 is a structure schematic diagram of structure for assembling motor end covers and housing of the invention. Detailed Description of the Invention In combination of the figures, the technical scheme of the invention is further specifically described through the following embodiments. Embodiment: As shown in Figure 1, a structure for assembling motor end covers and a housing comprises the front end cover 1 and the rear end cover 2, which are die-casted and molded by cast aluminum, as well as the aluminum alloy housing 4 which is coaxially clamped and fixed between the front end cover 1 and the rear end cover 2, four front end cover lugs 11 are integrally connected on the radial outer edge surface of the front end cover 1, the four front end cover lugs 11 are uniformly distributed along the circumferential direction of the front end cover 1, four rear end cover lugs 21 corresponding to the front end cover lugs 11 are integrally connected on the radial outer edge surface of the rear end cover 2, and the front end cover lugs 11 and the rear end cover lugs 21 are planar bosses with semi-circular cross sections, wherein the inner end surfaces of the front end cover lugs 11 are flush with the inner end surface of the front end cover 1, the inner end surfaces of the rear end cover lugs 21 are flush with the inner end surface of the rear end cover 2, axial reinforcing ribs 5 which extend outwards are respectively formed on two sides of the front end cover lugs 11 and the rear end cover lugs 21, threaded through holes are respectively formed at the middle parts of the four front end cover lugs 11, unthreaded holes are respectively coaxially formed on the rear end cover lugs 21 corresponding to the front end cover lugs 11, screw rods 3 are respectively connected and fixed by penetrating the unthreaded holes and the corresponding threaded through holes, the four screw rods 3 are distributed outside the housing 4 and parallel to the axial lead of the housing 4, and the front end cover 1 and the rear end cover 2 are clamped and fixed at two ends of the housing 4 through the four screw rods 3. When in installation, the front end cover 1 and the rear end cover 2 firstly push against the two ends of the housing 4 in parallel, the positions of the front end cover 1 and the rear end cover 2 are rotated so as to enable the four front end cover lugs 11 on the outer edge surface of the front end cover 1 to be flush with the four rear end cover lugs 21 on the outer edge surface of the rear end cover 2 in a one-to-one manner, be further respectively inserted into the screw rods 3 from the outer side of the rear end cover 2 and be tightened with the front end cover 1. The specific embodiment described in the text is only an example of the concept of the invention. Those of skill in the art of the invention can make various modifications or supplements or adopt the similar ways for substitution to the 3 described specific embodiment without deviating from the spirit of the invention or exceeding a scope defined in claims. 4