Description Tubular Motor with Brake Technical Field of the Invention The present invention relates to a motor, in particular to a tubular motor with a brake mechanism provided on an output shaft of a gearbox. Background of the Invention Due to small space occupied for mounting, compact structure and large torque, a tubular motor is applicable to a golf trolley as a power source which drives the two wheels of the golf trolley to rotate. To ensure the golf trolley to stop running and stay at that position immediately at the moment of power-off, a brake must be used to consume the inertia and prevent the golf trolley from moving on because of its inertia. However, at present, the tubular motor brake employs the way of electromagnetic braking which has a disadvantage of high cost. Summary of the Invention An object of the present invention is to provide a tubular motor with a brake mechanism which features low cost and good braking performance. It solves the problems such as high cost of the electromagnetic braking way employed by the existing tubular motors. The object of the present invention mentioned above is mainly solved by the following technical solutions. A tubular motor with a brake is provided, including a motor main body, a rotor shaft and a tubular housing; a gearbox is provided between the rotor shaft and the output shaft; a brake mechanism is provided on the output shaft, the brake mechanism including at least a rotating claw fixed to the output shaft mutually, a sliding claw which is sheathed outside 1 the output shaft and has relative movement with the output shaft in the axial direction, and a handle driving the sliding claw to move in the axial direction; and the rotating claw and the sliding claw are fitted with each other through end surface teeth. The brake mechanism is controlled manually by the handle. When the sliding claw and the rotating claw which are sheathed outside the output shaft are fitted with each other, due to the non-rotational sliding claw, the rotation of the rotating claw is stopped and the rotation of the output shaft is further stopped, so that braking is realized and the movement of a golf trolley because of the inertia of the output shaft is prevented. When the rotating claw is separated from the sliding claw, the rotating claw may rotate freely with the output shaft. Preferably, an annular block is provided at the smooth end of the output shaft near the sliding claw. A brake seat is provided outside the sliding claw and the annular block. A reset spring is provided between the sliding claw and the brake seat. A threaded hole is provided on the annular block. The handle is fitted with the threaded hole on the annular block. An orifice, roughly a semicircle, is provided between the sliding claw and the annular block, and inside the orifice provided with a steel ball which drives the sliding claw to move in the axial direction with respect to the annular block. Therefore, the present invention features low cost, good braking performance and compact structure. Brief Description of the Drawing Fig. 1 is a structure diagram of the present invention; and Fig. 2 is a partially enlarged diagram of position A in Fig. 1. Detailed Description of the Invention The technical solution of the present invention will be further described by embodiments as below with reference to the accompanying drawings. 2 Embodiment 1: As shown in Fig. 1, a motor main body 20 is provided within a tubular housing 21. A rotor shaft 10 extends from the two ends of the tubular housing 21. A gearbox 23 and a gearbox 25 are provided at the two ends of the tubular housing 21, respectively. A ball bearing 11 and an oil seal 12 are provided between the rotor shaft and a gearbox body 9. Each gearbox has an output shaft. The gearbox 23 and the gearbox 25 each include a gearbox body 9, a cover 1, a countershaft built-up gear 7 and an output shaft built-up gear 14. The countershaft built-up gear 7 is disposed on a countershaft 27, and the countershaft 27 is supported rotatably between a brake seat 45 and the gearbox body 9 by a ball bearing 6 and a ball bearing 8. A hobbing is provided on the rotor shaft 10, and the rotor shaft 10 is in meshed transmission with the countershaft built-up gear 7 by this hobbing portion. A countershaft hobbing is provided on the countershaft 27, and the countershaft 27 is in meshed transmission the output shaft built-up gear 14 by this countershaft hobbing. A framework oil seal 16, a corrugated gasket 5 and a ball bearing 15 are provided in turn, from outside to inside, between an output shaft 22 and the gearbox body 9, and a ball bearing 13 is provided between the inner ends of the gearbox body 9 and an output shaft 22. A brake mechanism 4 is provided on the output shaft 22 of one gearbox 23. A brake housing 3 is provided outside the brake mechanism 4, with one end of the brake housing resisted against the gearbox body 9 and the other end thereof against the cover 1. The cover 1, the brake seat 45 and the gearbox body 9 are connected to each other by a slotted cone pointed set screw 17. A ball bearing 18 and an oil seal 19 are provided between a cover 11 and the output shaft 22. The brake mechanism 4 includes a rotating claw 41 fixed to the output shaft 10 mutually, a sliding claw 42 which is sheathed outside the output shaft 10 and has relative movement with the output shaft 10 in the axial direction, and a handle 43 driving the sliding claw 42 to move in the axial direction. The rotating claw 41 and the sliding claw 42 are fitted with each 3 other through end surface teeth. The rotating claw 41 connects and locates with the gearbox output shaft 22 in circumferential direction and in axial direction by a slotted cone pointed set screw 2. An annular block 44 is provided at the smooth end of the output shaft 10 near the sliding claw 42. A brake seat 45 is provided outside the sliding claw 42 and the annular block 44. A reset spring 48 is provided between the sliding claw 42 and the brake seat 45. A hole 441 is provided on the annular block 44. The handle 43 is fitted with the brake seat 45 by threads, and then into the hole 441 of the annular block. An orifice 46, roughly an oval, is provided between the sliding claw 42 and the annular block 44, and inside the orifice provided with a steel ball 47 which drives the sliding claw 42 to move in the axial direction with respect to the annular block. When a handle connects with a annular block by threads, the annular block bears the radial pressure from the end of the handle, resulting in that a steel ball located within a circular orifice between the annular block and a sliding claw bears the pressure toward the external surface of the annular block to drive the sliding claw to move in the axial direction with respect to the annular block, and then to fit with a rotating shaft. Due to the non-rotational sliding claw, the rotation of the rotating claw is stopped, and the rotation of the output shaft is further stopped, which has the effect of braking, so that the moving on of a golf trolley because of the inertia of the output shaft is prevented. When the radial pressure force applied at the end of the handle disappears, the sliding claw resets under the effect of a reset spring, and the rotating claw separates from the sliding claw, and then the claw may rotate freely. 4