Uncle Sam pays on average $2,224 per month in Social Security disability insurance benefits to about 8.3 million Americans.

If, for whatever reason, you need to apply for SSDI, there are a few things you need to know.

It's hard to get

For starters, it’s “very” hard to get, said Andy Landis, author of "Social Security: The Inside Story."

“Social Security has a very strict definition of disability,” he said.

Social Security considers you disabled if all of the following are true:

• You cannot do work that you did before because of your medical condition.

• You cannot adjust to other work because of your medical condition.

• Your disability has lasted or is expected to last for at least one year or to result in death.

“Basically, you have to be almost totally disabled for an extended time period,” Landis said.

You have to have at least a part-time work history to get SSDI, Landis said.

In general, to get disability benefits, you must meet two different earnings tests, according to Social Security: a recent work test, based on your age at the time you became disabled, and a duration of work test to show that you worked long enough under Social Security.

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There are backups if you don't have the work history or if your SSDI payment is low.

“Backups are available for disabled widows and widowers and for disabled adult children,” Landis said.

If you're low-income, you should check out the Supplemental Security Income (SSI) program, which provides monthly payments to adults and children with a disability or blindness and who have income and resources below specified amounts.

It can take a year to process a claim

Landis noted that it could take a year to process your claim. If you have enough work to qualify for disability benefits, Social Security uses a step-by-step process involving five questions to determine if you are disabled.

The list of questions starts with whether you are working. If your earnings average more than $1,310 a month, you generally cannot be considered disabled. If you are not working, Social Security will send your application to the Disability Determination Services office that will make the decision about your medical condition.

Planning opportunities

William Reichenstein, co-author of "Social Security Strategies: How to Optimize Retirement Benefits," said there are some little-known financial planning opportunities for those collecting SSDI.