There is a misconception by some schedulers that it is better if you plan using only fixed crew composition. Also most software cannot handle the true variable crew models that will yield better schedules adding to the poor resource planning practices. Still are relatively good in the modeling of the simple case of a fixed composition crew.

If this is what you want to model then it is just a matter of fixing your resource quantities and workloads.

The following example is based on two cranes availability. There are no logic links among the activities but a resource to be shared when available. Activity 1 need the crane 6 hours per day or 75%, the same goes for Activity 2 while activity 3 needs a crane 4 hours per day.

Note that assigning a crane 4 hours per day is not the same as 2 cranes 2 hours per day or four cranes 1 hour per day, each would represent different models. 

By using adequate quantity and workload you shall be able to get good resource leveling that will prevent assigning too many tasks to the cranes.

It is very easy once you get it, it is just that usually you do not assign a labor resource to several activities in most construction activities while equipment is frequently shared among activities in parallel. In other job types it is very common to share labor resources on different activities the same day.

Assigned quantity is usually expressed using units and workloads, say one crane [quantity/units] 2 hours per day [workload].

Productivity and work are different and are better expressed using physical measure of work per unit time in case of productivity and volume of work in case of work.

A typical example of equipment workload would be when sharing a piece of equipment among several activities, say a crane shared one hour per day on 12 activities using an 8 hour calendar, a maximum of 8 such activities will be assigned on a single day and remaining will be delayed until the crane is available to work required hours on the remaining activities. 

Some schedulers assign the crane to a hammock activity but this would not correctly resource level the crane usage and might allow for all activities be scheduled during the same day. Others try to assign such equipment using fractions or percentage of equipment but this would allow wrong assignments. Say you have 3 activities that need a crane 5 hours a day and you have 2 cranes. Only two activities can be worked on the same day but the wrong model might allow the 3 activities to be scheduled the same day by assigning unintended distribution of resource hours. Such model might assign Crane 1 five hours to Activity 1, Crane 2 five hours to Activity 2 and the two cranes 2.5 hours to Activity 3 to make the 5 equipment hours. The model would not make this transparent to the scheduler. In the case of five cranes available it might assign five cranes at the same time to a single activity. This can yield impossible assigments such as when a plane can get you from A to B in four hours, then two planes will get you in 2 hours. 

It might look complicated but using good software that provides the correct metrics for such assignments makes it easy once you get used to it, otherwise would be very difficult, impossible or would yield erroneous models. 

It is very common to assign all resources to a fixed Crew, that will result in feasible schedule but not necessarily the most efficient resource planning. Say a crew requires 10 units of Resource A but only 9 are available and the 10 th will be available in 6 weeks, it does not make sense to delay the activity 6 weeks and keep the nine available resources idle. In practice the activity will be started when enough are available, even when total activity duration will be increased it will end way before than if you wait the six weeks. Poor models will not be able to account for this and in the case of hundreds of activities the planner might not notice the opportunity lost. 

Still in your fixed crew if the equipment is a crane to be shared among several activities you shall consider use of partial assigments or workloads.

About unit of measure we use a unit of measure related to the volume of work. For an excavator we use CM to quantify volume of work, we use CM/Hour to quantify productivity. For asphalt pavement we use tons to quantify volume of work, we use Tons/Hour to quantify productivity.

Machine hours represent effort. Keep in mind different pieces of equipment might produce different amount of work within the same equipment hours. 

For serious resource planning at minimum your software shall differentiate between duration, effort and volume of work. It shall provide good resource leveling algorithms and advanced resource modeling such as skills, multi resources as skills, variable resource quantity and workload that will reduce idle resources in the same way job manager will attempt to do it at the job site. Unfortunately most software with regard to resource planning are a joke and do not provide the tools for serious resource models.