Wheelbase for movable structure

1. A wheelbase for connecting front wheels to rear wheels, comprising:

a first frame unit connected to the front wheels, wherein a top side of the first frame unit is open;

a second frame unit connected to the rear wheels, wherein the second frame unit is positioned above the first frame unit and slidably coupled to the first frame unit so as to cover the top side of the first frame unit, and wherein a space is formed between the first frame unit and the second frame unit;

a slider unit positioned in the space and coupled to the second frame unit;

a driving unit positioned at one end of the first frame unit, wherein the driving unit generates a driving force; and

a driving-force-transmitting unit connected with the driving unit and the slider unit, wherein the driving-force-transmitting unit transmits the driving force to the slider unit,

wherein the driving unit moves the slider unit in a lengthwise direction of the first frame unit so as to adjust a distance between the front wheels and the rear wheels,

wherein the driving-force-transmitting unit includes a threaded rod extending in the lengthwise direction of the first frame unit and passing through the slider unit to engage with the slider unit,

wherein the driving unit rotates the driving-force-transmitting unit to linearly move the slider unit along the driving-force-transmitting unit,

wherein a plurality of balls coupled to the slider unit are arranged between the threads of the threaded rod,

wherein the first frame unit comprises rail grooves formed on both sides of the first frame unit and extending in the lengthwise direction of the first frame unit,

wherein the second frame unit comprises rail protrusions formed on inner surfaces of the second frame unit, the rail protrusions being inserted into the rail grooves and configured to slide along the rail grooves, and

wherein a to surface of the second frame unit extends towards the front wheels to ensure covering the top side of the first frame unit during a sliding movement of the second frame unit.

2. The wheelbase according to claim 1 , wherein the driving unit comprises a motor-drive unit for generating torque; and

wherein the wheelbase includes a decelerating unit connecting the motor-drive unit with one end of the driving-force-transmitting unit and transmitting the torque of the motor-drive unit to the driving-force-transmitting unit.

3. The wheelbase according to claim 1 , further comprising switching units positioned at both ends of the space, wherein the slider unit moves between the switching units, and wherein when the switching units contact the slider unit, the driving unit stops.

4. The wheelbase according to claim 1 , wherein the second frame unit comprises:

a pipe-fixing unit covering part of a wheel pipe connected to the front wheels or the rear wheels and fixing the wheel pipe to at least one of the first frame unit or the second frame unit; and

a buffer unit made of elastic materials, wherein the buffer unit is positioned between the pipe-fixing unit and the wheel pipe.