门机金属结构是由哪几部分组成的？
参考答案：
主要由以下几部分组成（以M2533门机为例）：
1、臂架系统：臂架系统为四连杆系统。它包括起重臂、象鼻梁、大拉杆三大件；
2、立柱臂架平衡系统：立柱臂架平衡系统包括立柱上、下段、平衡梁、配重及平衡小拉杆等，立柱上下段分别由两片工字型焊接板梁组成的框架构成。
3、转盘：转盘是门座起重机的主要结构，用来支承起升机构、变幅机构、旋转机构驱动装置、臂架系统、立柱臂架平衡系统、机器房、电气控制柜、司机室等部件。
4、门架：门架结构是整个起重机的基础结构，支承着门座起重机的所有上部结构，它是门座起重机的重要部件。
Taking a Nap During the Day Medical experts say most Americans do not get enough sleep. They say more Americans need to rest for a short period in the middle of the day. continuing with other activities. One study earlier this year found that persons who for a few minutes during the day were less likely to die of heart disease. The study more than 2,300 Greek adults for about six years. Adults who rested for half an hour at least three times a week had a 37 percent risk of dying from heart disease than those who did not nap. The study organizers said the strongest evidence was in working men. They said naps might improve by mitigating(减低) tension caused by work. Some European and Latin American businesses have supported the of napping for many years. They urge people to __ work, go home and have a nap before returning. In the United States, some companies let workers rest briefly in their offices. They believe this reduces and accidents, and _increases the amount of work a person can do. Sleep experts say it is likely that people make more mistakes at work than at other times. They say people should not important duties when they feel sleepy. And they say the best thing to do is to take a nap. About twenty minutes of rest is all you need. Experts say this provides extra energy and can increase your effectiveness _ the end of the day. But experts _that a nap should last no more than twenty to thirty minutes. A longer nap will put the body into deep sleep and waking up will be difficult.
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