How is the zakat calculated for rented excavation machinery, the rent for which is not paid regularly in terms of quantity and quality, bearing in mind that it was sold and a new one purchased, and what are the resulting expenses for maintenance and spare parts?

If you bought the excavator with the intention of renting it out, not selling it, then there is no Zakat (definition) due on it. As for its rent, Zakat is obligatory on each partner if his share reaches the nisab (definition) and a year has passed since it came into his possession, upon the tenant’s full utilization of the benefit. If the company delays payment of the rent after it becomes due, the hawl (one year period for Zakat calculation) begins from the time it became due, and the rent is subject to Zakat upon its receipt for the past years.