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).
When to use the simple past tense. We use the simple past tense for actions that began and finished in the past. We do not have to say exactly when. The important thing is that it happened and is not happening now. 'I saw it' When to use the past participle. The past participle is used with the following tenses: Present Perfect. I have seen it. Past Perfect