Học Chi Tiết Máy Bài 165:Tính lò xo chịu kéo, nén

Tính lò xo chịu kéo, nén:

Xét lò xo chịu tải như trên Hình 22-8:

hoc-chi-tiet-may-75-8

Dưới tác dụng của lực F, dây lò xo bị xoắn bởi mô men xoắn T = F.D/2. Do dây bị uốn cong, ứng suất xoắn ở biên trong của dây lớn hơn biên ngoài. Mô men chống xoắn của tiết diện dây lò xo: W0 = π .d3/16

– Ứng suất xoắn trên dây lò xo được xác định theo công thức:

hoc-chi-tiet-may-75-9

Trong đó k là hệ số kể đến độ cong của dây lò xo. Giá trị cùa k được tính theo công thức:

hoc-chi-tiet-may-76-1

– Ứng suất cho phép [τ x] được xác định như sau:

x] = (0,4÷0,5).σb .

Đối với các thép thường dùng làm lò xo, lấy σb = 1500 ÷ 1700 MPa.

Kiểm tra độ bền của lò xo bằng cách:

-Tính τx, theo công thức 22-1.

-Xác định ứng suất cho phép [τx].

-So sánh τx và [τx], rút ra kết luận. Nếu τx =< [τx], lò xo đủ bền.

Thiết kế lò xo được tiến hành như sau:

+ Xác định ứng suất cho phép [τx],

+ Giả sử điều kiện τx =< [τx], rút ra được công thức tính đường kính dây lò xo

hoc-chi-tiet-may-76-2

+ Số vòng làm việc của lò xo được tính theo công thức:

hoc-chi-tiet-may-76-3

Trong đó:

Fmax là lực lớn nhất tác dụng lên lò xo,

Fmin là lực nhỏ nhất tác dụng lên lò xo,

λmax là chuyển vị lớn nhất của lò xo, khi chịu Fmax,

λmin là chuyển vị nhỏ nhất của lò xo, ứng với Fmin.

 

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