10: N ou cung dung, ph on loai, ctao, nguy on l y làm vi ec, xoc d inh cac thong so chinh cua cac loai may dam be tong.

MAY DAM BE TONG

Muon tao duoc cau kien be tong vung chac thi sau khi do be tong chung ta can tim cach nen chat no lai chu yeu bang rung dong va ta goi chung la qua trinh dam.

1. Tac dung cua rung dong

Trong cong nghiep san xuat cac cau kien be tong va be tong cot thep, de lam chat hon hop be tong nguoi ta thuc hien bang cac phuong phap sau: rung dong, ep hoac quay li tam v.v... trong do rung dong duoc su dung rong rai nhat vi thiet bi don gian, pham vi ap dung rong, hieu qua lam chat cao va thich hop voi nhieu loai cau kien xay dung.

Duoi tac dung cua rung dong, cac hat cot lieu cua hon hop be tong duoc truyen nang luong va chuyen dong voi van toc khac nhau. Khi nay lien ket giua cac hat bi pha vo, noi ma sat giam, hon hop tro nen luu dong hon, khong khi va nuoc duoc day ra ngoai va cac hat tien sat lai gan nhau hon, lam cho khoi luong the tich cua hon hop be tong tang len (toi 1.61.65 lan) va khuon duoc dien day tot hon.

2. Cac phuong phap dam be tong

Phu thuoc vao phuong truyen rung vao hon hop be tong ma co cac phuong phap dam nhu sau: rung be mat; rung dat trong va may rung the tich.

* May rung be mat: Nguon dao dong dat truc tiep len be mat tren cua hon hop be tong va dao dong duoc truyen tu ben tren vao trong long cua hon hop be tong luu dong voi chieu day lop dam khong lon (<200mm). (nguon dao dong o day co the la ban rung vo huong "quy dao tron hoac e lip" va ban rung dinh huong "quy dao thang dung hoac huong ngang").

* May rung dat trong: duoc tha chim sau trong long hon hop be tong va rung dong duoc truyen tu ben trong ra xa xung quanh. Phuong phap nay rat co hieu qua cao. Thuong duoc dung de dam be tong dang khoi, cot v.v...(nguon dao dong o day co the la dao dong dieu hoa, khong dieu hoa, hoac dao dong nhieu tan so).

* May rung the tich: hon hop be tong duoc dua vao khuon cung. Dao dong duoc truyen qua khuon (vo hoac cot pha) toi toan bo the tich cua hon hop.

Ngoai ra de tang cuong hieu qua lam chat hon hop nguoi ta su dung ket hop cac phuong phap tren hoac va - rung; hoac rung - ep; hoac can - rung.

3. Cau tao cua may dam thong dung

a- Cau tao cua may dam mat (dam ban)

May dam ban la mot trong cac loai dam ngoai, duoc dung de dam cac khoi be tong co dien tich rong nhu nen nha, san nha, nen duong.

Cau tao va nguyen ly lam viec cua may dam ban duoc the hien nhu tren hinh 9-9: Bo phan gay rung la dong co dien co vo 1, ro to 2, bi do 3, khoi lech tam 4, ban dam 5. Khi dong dien, truc dong co quay, tren do

co bo phan lech tam quay theo. Khoi lech tam quay gay ra luc ly tam, lam ban dam dao dong, thuc hien qua trinh dam.

May dam ban:1- Vo dong co; 2- Ro to; 3- Vong bi; 4- Khoi lech tam; 5- Ban dam.

b- Cau tao cua may dam trong (dam dui)

* Dam dui truc mem: Voi loai dam nay chung ta co ba loai chinh: dam dui truc lech tam (it dung), dam dui lac trong (tiep xuc ngoai) va dam dui lac ngoai (tiep xuc trong). Hien nay thuong su dung cac loai may dam dui truc mem duong kinh ngoai cua chay la 25, 35, 50mm tuong ung voi chieu dai cua truc mem la 2, 3,5 va 5m.

Truc 1 duoc dan dong tu dong co, truyen chuyen dong toi truc 3 qua khop 2. Khoi lech tam 5 lap tren tuc 3, lan tren be mat chi tiet 6 lap tren vo dam 4 va gay ra chan dong cho qua dam, thuc hien viec dam be tong. Cung co the lap truc lech tam tren hai o bi va gay rung.

Tuy theo ty le duong kinh hai be mat lan tron (D/d) va duong kinh vo qua dam, tan so dao dong cua dam n=600024000 lan/ph, duong kinh vo nho co tan so lon.

Nhuoc diem cua dam dui truc mem la ma sat giua truc va vo rat lon nen hao ton cong suat lon, truyen dao dong khong xa.

* Dam dui can cung:

Dong co va bo gay rung deu dat trong vo qua dam, day dan dien tu ngoai vao duoc luon qua can cung 5 de dieu khien dong co, lam quay truc dong co gay rung, thuc hien qua trinh dam.

Loai nay khac phuc duoc cac nhuoc diem cua dam dui truc mem, nhung nang nen thuong phai dung phuong tien nang de nang qua dam. Dam dui can cung co uudiem la hieu suat truyen luc cao, vi khong dung truc mem nen tuoi tho cua may tang cao. No co the lam viec voi be tong kho (do sut tu 13cm), co the lien ket hang loat lam viec dong thoi nen rat phu hop de dam khoi luong be tong lon co cot thep tuong doi thua.

May dam dui truc mem: a- Lac trong; b- Lac ngoai; c- So do truyen dao dong khi dam; 1- Truc mem; 2- Khop noi; 3- Truc quay; 4- Vo; 5- Khoi lech tam; 6- Vo lan.

May dam dui can cung:1- Vo; 2- Truc lech tam; 3- O bi; 4- Dong co dien; 5- Can cung; 6- Cong tac dien.

4. Cac thong so co ban cua may dam

a. Pham vi dam huu ich cua may dam mat (xac dinh chieu sau dam h):

Pham vi dam huu ich cua may duoc xac dinh qua cong thuc sau:

Trong do: Go- trong luong qua vang;

Gd¬- trong luong may dam;

e- do lech tam cua qua vang;

a- gia toc can thiet de thay doi do nhot cua hon hop, a= 57m/s2;

- van toc goc;

Gbt- trong luong be tong duoc dam, Gbt = btVbt= bt lbh

voi: bt- trong luong rieng cua hon hop be tong;

l, b - chieu dai va chieu rong cua may dam;

h - chieu sau dam.

Thay vao tren ta co:

Theo kinh nghiem h  300mm. Tot nhat la h=150mm. Do vay neu ket cau qua day chung ta phai phan lop de dam.

b. Pham vi dam huu ich cua may dam trong (xac dinh chieu sau dam va ban kinh tac dong)

Chu yeu dua vao thuc nghiem

- Chieu sau dam huu ich: tot nhat la bang chieu dai cua dam (chay), hoac it nhat la bang 2/3 chieu dai cua dam.

- Ban kinh tac dong huu ich:

+ Voi ket cau be tong lien khoi: R=0.50.6m;

+ Voi ket cau be tong duc san: R=0.20.4m;

c. Nang suat cua may dam

- Nang suat cua may dam mat:

, m3/h

Trong do:

F - dien tich mat ban dam, m2;

h - chieu sau dam huu ich, m;

td- thoi gian dam o vi tri , td = (2530)s;

t - thoi gian thay doi vi tri;

k - he so su dung may, k = (0,60,7);

- Nang suat cua may dam dui:

, m3/h

Trong do:

R - ban kinh tac dong huu ich, m;

h - chieu sau dam huu ich, m;

td- thoi gian dam o vi tri , td = (2530)s;

t - thoi gian thay doi vi tri, s;

k - he so su dung may, k = (0,60,7).