问题 问答题 论述题

试述齿轮的点蚀现象。

答案

参考答案:

齿轮的点蚀是齿轮传动的失效形式之一。即齿轮在传递动力时,在两齿轮的工作面上将产生很大的压力,随着使用时间的增加,在齿面便产生细小的疲劳裂纹。当裂纹中渗入润滑油,在另一个轮齿的挤压下被封闭的裂纹中的油压力就随之增高,加速裂纹的扩展,直至轮齿表面有小块金属脱落,形成小坑,这种现象被称为点蚀。轮齿表面点蚀后,造成传动不平稳和噪声增大。齿轮点蚀常发生在闭式传动中,当齿轮强度不高,且润滑油稀薄时尤其容易发生。

单项选择题
阅读理解

阅读理解。

    Surveys of American teenagers find that about half of them do not get enough sleep on school nights. They

get an average of 60 to 90 minutes less than experts say they need. 

    One reason for this lack of sleep is biology. Experts say teens are biologically programmed to go to sleep

later and wake up later than other age groups. Yet many schools start classes as early as seven in the morning.

As a result, many students go to class feeling like sixteen-year-old Danny. He plays two sports, lacrosse and

football. He is an active teen-except in the morning.

    DANNY:"Getting up in the morning is pretty terrible. I'm just very out of it and tired. And then going to

school I'm out of it, and through first and second period I can barely stay awake.

    "Michael Breus is a clinical psychologist with a specialty in sleep problems. 

    Teens, he says, now commonly sleep for only 6.5 hours to 7 hours. He says sleepy teens can experience

a form of depression (压抑) that could have big effects on their general well-being. It can affect not just their

ability in the classroom but also on the sports field and on the road. 

    So what can schools do about sleepy students? The psychologist says one thing they can do is start classes

later in the morning. Eric Peterson is the head of St. George's School in the northeastern state of Rhode Island.

He wanted to see if a thirty-minute delay would make a difference. It did. 

    He says visits to the health center by tired students decreased by half. Late arrivals to first period fell by a

third. And students reported that they were less sleepy during the day. 

    Eric Peterson knows that changing start times is easier at a small, private boarding school like his. But he is

hopeful that other schools will find a way. 

    Patricia Moss, an assistant dean at St. George's School, says students were not the only ones reporting

better results. 

    PATRICIA MOSS:"I can say that generally all the teachers noticed immediately that kids were happier to be

there at eight-thirty than they were at eight."

1. According to experts like Michael Breus, how many hours of sleep should teens get every day? [ ]

A. 6.5 hours to 7 hours

B. at least 7.5 hours

C. More than 9 hours

D. Less than 7 hours

2. According to some experts, why do teens wake up later than adults?

A. Because they are lazier.

B. Because they need more sleep.

C. Because they are biologically programmed to sleep late and wake up late.

D. Because they play sports.

3. The article mentions the story of sixteen-year-old Danny to _____.

A. show how he hates getting up in the morning.

B. show how teens can be affected by a lack of sleep.

C. show how he turns from an active boy to a tired school child.

D. show how teens hate school.

4. What does Eric Peterson do about the problem of sleepy students?

A. He visits the health center.

B. He punishes late students.

C. He punishes sleepy students.

D. He starts his school time 30 minutes later.

5. Which of the followings is not true according to the article?

A. Later school time leads to fewer late arrivals.

B. Er ic Peterson’s experiment h as answered his question.

C. The writer of this article seems to support later school time.

D. Teachers of St. George’s School don’t seem to support later school time.