End face mechanical seal - Seal categories. 1  All mechanical seals must contain the four elements described above but the way those functional elements are arranged may be quite varied. 2  Mechanical seals are generally classified into two main categories: Pusher or Non-Pusher.
The past modal perfect tense begins with any past tense modal (could, should, would, might) followed by âhave,â plus a past tense action verb (regular verbs ending with â-edâ and many irregular verbs ending with-en).
English Grammar Rules. The simple present tense in English is used to describe an action that is regular, true or normal. We use the present tense: 1. For repeated or regular actions in the present time period. 1  I take the train to the office. 2  The train to Berlin leaves every hour. 3  John sleeps eight hours every night during the week.