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).
The first for is grammatically correct. When you use the past simple form of the verb 'DO' the the verb which follows remains in the infinitive form. For example, we say ' He did walk' and NOT ' He did walked'.