A cylinder unit includes a cylinder, a rod guide, a piston, a piston rod, and a rebound member provided between the rod guide and the piston to absorb impact generated when the cylinder unit is fully extended. The rebound member includes a coil spring arranged around an outer circumference of the piston rod and a pair of upper and lower holder members fitted to the fixing portions formed in both ends of the coil spring so that the piston rod is inserted into a shaft center hole. The center portion formed between the fixing portions of the coil spring has a diameter smaller than that of the fixing portion.
1. A cylinder unit, comprising:
a cylinder;
an annular rod guide provided in one end side of the cylinder;
a piston having an outer circumference slidably adjoining an inner circumference of the cylinder;
a piston rod that is movably inserted into the cylinder by interposing the piston so as to pass through the rod guide; and
a rebound member provided between the rod guide and the piston to absorb impact generated when the cylinder unit is fully extended,
wherein
the rebound member comprises
a coil spring arranged around an outer circumference of the piston rod so as not to make contact with the inner circumference of the cylinder, the coil spring being a circular cross-sectional wire, and
an upper holder member and a lower holder member fitted to fixing portions formed in both ends of the coil spring, wherein the piston rod is inserted into a shaft center hole of each of the upper holder member and the lower holder member,
when the cylinder unit is fully extended, the coil spring is fully compressed and does not make contact with the inner circumference of the cylinder,
the upper holder member is slidable along the outer circumference of the piston rod and the lower holder member is fixed to the outer circumference of the piston rod, and
the coil spring has a center portion formed between the fixing portions of the coil spring, and the center portion has a diameter smaller than that of the fixing portions.
2. The cylinder unit according to claim 1 ,
wherein the coil spring includes the fixing portions, diameter-reducing portions whose diameters are gradually reduced from the fixing portions, and the center portion formed between the diameter-reducing portions.
3. The cylinder unit according to claim 2 , wherein
the center portion is formed across a wide area between the upper holder member and the lower holder member.
4. The cylinder unit according to claim 2 , wherein
each of the upper holder member and the lower holder member has a guide portion fitted to an inner circumference of the corresponding fixing portion,
a diameter of an outer circumference of a leading end of the guide portion of each of the upper holder member and the lower holder member is gradually reduced, and
each of the diameter-reducing portions of the coil spring is set not to make contact with the leading end of the corresponding guide portion.
5. The cylinder unit according to claim 4 , wherein
the diameter-reducing portion of the coil spring is formed in a tapered shape and the leading end of the guide portion is formed in a tapered shape, and
a taper of the tapered shape of the diameter-reducing portion of the coil spring and a taper of the tapered shape of the leading end of the guide portion have different inclination angles.
6. The cylinder unit according to claim 2 , wherein
the upper holder member includes a first diameter-reducing portion having a diameter gradually reduced toward the lower holder member,
the first diameter-reducing portion is located within, without contacting, one of the diameter-reducing portions of the coil spring,
the lower holder member includes a second diameter-reducing portion having a diameter gradually reduced toward the upper holder member, and
the second diameter-reducing portion is located within, without contacting, another one of the diameter-reducing portions of the coil spring.
7. The cylinder according to claim 1 , wherein
the coil spring includes a plurality of coils,
when the coil spring is fully compressed,
the coils neighboring along an axial direction of the coil spring contact each other, and
one of the plurality of coils is shifted in a radial direction of the coil spring to abut the piston rod without making contact with the inner circumference of the cylinder.
8. The cylinder unit according to claim 1 , wherein
a distance between the center portion of the coil spring and the piston rod is smaller than 1 mm.
9. A cylinder unit, comprising:
a cylinder;
an annular rod guide provided in one end side of the cylinder;
a piston having an outer circumference slidably adjoining an inner circumference of the cylinder;
a piston rod that is movably inserted into the cylinder by interposing the piston so as to pass through the rod guide; and
a rebound member provided between the rod guide and the piston to absorb impact generated when the cylinder unit is fully extended,
wherein
the rebound member comprises
a coil spring arranged around an outer circumference of the piston rod so as not to make contact with the inner circumference of the cylinder, the coil spring being a circular cross-sectional wire, and
an upper holder member and a lower holder member fitted to fixing portions formed in both ends of the coil spring, wherein the piston rod is inserted into a shaft center hole of each of the upper holder member and the lower holder member, and each of the upper holder member and the lower holder member has a guide portion fitted to an inner circumference of the corresponding fixing portion,
when the cylinder unit is fully extended, the coil spring is fully compressed and does not make contact with the inner circumference of the cylinder,
the coil spring has a center portion formed between the fixing portions of the coil spring, and the center portion has a diameter smaller than that of the fixing portion,
the coil spring further includes diameter-reducing portions whose diameters are gradually reduced from the fixing portions,
a diameter of an outer circumference of a leading end of the guide portion of each of the upper holder member and the lower holder member is gradually reduced, and
each of the diameter-reducing portions of the coil spring is set not to make contact with the leading end of the corresponding guide portion.
10. The cylinder unit according to claim 9 , wherein
the diameter-reducing portion of the coil spring is formed in a tapered shape and the leading end of the guide portion is formed in a tapered shape, and
a taper of the tapered shape of the diameter-reducing portion of the coil spring and a taper of the tapered shape of the leading end of the guide portion have different inclination angles.